Why Krishn Dies at 16th Parv in MBH
This article is curtailed from the article   "Krishn, last days: why Vyaas kills Him at The 16th Parv" - by Indrajit Bandhyopadhyay.

Yaagyavalkya Jee says ----
One Year = Brihatee Chhand (Meter) = 12 Full Moon + 12 eight days (of the fortnight of waning Moon) + 12 New Moon = 36 = 36 syllables of Brihatee Chhand.
Here again, we get the figure "8".

He mentions a Super Year that comes after 15 Years - that is, the 16th year - at the completion of 15 Years = 360 Full Moon + 360 New Moon = (360 Days x 15 Years)
Again, we get the figure 36.

The Paandav reach 36 years of rule at the 16th Parv - thus 36 merges with 16.
Shatapath Braahman states that Anushtup Chhand (16 x 2 = 32 syllables - 16, 8 x 4) cannot take one to God - it can take one only up to some distance - while Brihatee Chhand can one to God.

17 comes after 16 - the Pandavas leave for their final journey at the 17th Parv - and it cannot be an accident then, that at the end of 17th Parv Yudhishthir calls Draupadee "Brihatee" - in fact, preparing for the 18th Parv - and 18 is half of 36 syllables of Brihatee Chhand syllables.

In Yajur Ved (2.1.4;  and   5.4.5), the 16 portions of Jeev are Vritra’s coils. Vyaas Jee teaches to his son Shuk that 16 elements constitute the "body of time" - Kaal Shareerah (MBH, 12.309.24-25).

Even before, Vashishth defines Moksh to Karaal Janak that when the 16 portions of Jeev are no more united with Prakriti, it is called Moksh (MBH, 12.293.7). The 16th portion of Jeev is subject to no modification (MBH, 12.293.1-11). The eternal and immutable 16th portion is called Som.

Maarkandeya Jee says that in Kali Yug, the 16th year is the age in which "men are overtaken with decrepitude and decay and the period of life itself is soon outrun’ (MBH, 3.186.52), which is symbolically represented through Abhimanyu’s death at the age of 16.

Thus when Vyaas Jee destroys the Vrishni Vansh in the 16th Parv, he suggests constancy in human affairs even when power changes hand or is destroyed; it is as if, the destruction of Vrishni Vansh is no big matter in the scheme of human affairs.

Another example of 16 - In Shaanti Parv, Yudhishthir is riding bullock-cart drawn by 16 white bullocks and Dhritraashtra rides palanquin.

No 16 in Brihadaaranyak Upanishad

The significance of '16' can be best understood with reference to Brihadaaranyak Upanishad. Brihadaaranyak Upanishad explains the mystery of 16 as follows:
"This Prajaapati (Hiranyagarbha) has sixteen digits and is represented by the year. The nights (and days) are his fifteen digits, and the constant one is his 16th digit. He (as the Moon) is filled as well as wasted by the nights (and days). Through this sixteenth digit he permeates all these living beings on the New-Moon night and rises the next morning. Therefore on this night one should not take the life of living beings, not even of a chameleon, in adoration of this deity alone (1.5.14). That Prajaapati who has sixteen digits and is represented by the year is indeed this man who knows as above. Body or Wealth constitutes his fifteen digits, and the Aatmaa (Soul) is his sixteenth digit. He is filled as well as wasted by Body or wealth. This Aatmaa (Soul) stands for a nave, and Body or wealth is the felloe. Therefore if a man loses everything, but he himself lives, people say that he has only lost his outfit (1.5.15)."

What is suggested here
(1) Prajaapati’s Samvatsar (One Year) = 16 Kalaa
(2) 15 Kalaa (of these) = Raatri (Night) = Vitta (Body Wealth) = Subject to Kaal (Time)
(3) The 16th Kalaa is Dhruv (Constant), that is Aatmaa (Soul) = and is Beyond Kaal (Time) = Beyond the transitory Rotation of Day (Day and Night) and Time
(4) The 15 Kalaa represent Duality of Growth and Decay – that is Transitory Existence; the Bodily existence is part of this transience
(5) The 16th Kalaa or Aatmaa is the Nave, and the Body Wealth is the Felloe in this Wheel of Time
(6) "Through this sixteenth digit he permeates all these living beings on the New-Moon night and rises the next morning - so 'maavaasyaam raatrim etayaa shodasyaa kalayaa sarvam idam praanabhrid anupravishya tatah praatar jaayate”: Amaavasyaa (New Moon) is the end of 16 Kalaa.
(7) Though here only Raatri (Night) is mentioned with reference to the Moon, the Rotation of Day and Night is implied – that is, the 16 Kalaa involve Rotation of Day and Night till it completes this phase on the 16th Night or 16th Kalaa of Amaavasyaa (New Moon).
(8) Since Amaavasyaa (New Moon) is mentioned, the Waning Moon from Poornimaa (Full Moon) is implied
(9) One who knows and understands all above is Purush and Prajaapati – that is an Ordinary Man can transform himself into true Purush and Prajaapati by realizing the relationship of Kaal (Time) and Enlightenment, or the Wheel of Time – the true significance of Kaal Chakra

No 16 in Chhaandogya Upanishad

In Chaandogya Upanishhad, Uddaalak tells his son Shwetketu:

“VI-vii-1: "Dear boy, man consists of sixteen parts. Do not eat for fifteen days; drink as much water as you like. Praan is made up of water, and the Praan of one who drinks water is not cut off.
VI-vii-2: Shwetketu did not eat for fifteen days. Then he approached him saying, "What shall I say?"
The father said - "The Riks, the Yajus, and the Saams, dear boy."
"They do not at all arise in me, Sir."
VI-vii-3: The father said to him - "Dear boy, just as a single ember of the size of a firefly, left over from a large burning fire, cannot burn any more than that, even so, dear boy, of your sixteen parts only one part is left over, now by means of that you cannot perceive the Ved. Eat, then you will understand me."
VI-vii-4: He ate and then approached his father. Whatever he asked him, he answered them all.
VI-vii-5-6: The father said to him - "‘Dear boy, just as when a single ember of the size of a firefly left over from a large burning fire, is made to blaze up by adding straw and it burns much more than before, even so, dear boy, of your sixteen parts, only one part remained, and that being nourished by food, has been made to blaze up; and by that you perceive the Ved now. Hence, dear boy, the mind is made up of food, the Praan is made up of water, and speech is made up of fire."
From his words, (Shwetketu) understood it – yea, he understood it.”

What are suggested here are –

(1) Man consists of 16 Kalaa
(2) Realization of Ved is possible on the 16th Kalaa
(3) 16 suggests Enlightenment that reveals the true meaning of Ved
(4) However, without proper nourishment of the 15 Kalaa with Food, Ved do not reveal on the 16th; thus, the 15 Kalaa are as important as the 16th - that is, the 16 Kalaa together are an Existential Reality, and the 16th can be reached only through the 15 Kalaa
(5) Mind = Water; Praan = Food
(6) The 16 Kalaa needs nourishment of both Mind/Water and Praan/Food

No 16 in Shatapath Braahman

The same idea is contained in Shatapath Braahman (10:4:1:18):

"These sixteen digits convey the food to that vital air; and when they take to conveying no food to it, then it consumes them and departs (from the body): hence he who is hungry here, feels very restless, consumed as he is by his vital airs; and hence he who suffers from fever becomes very thin, for he is consumed by his vital airs.”

The Waning Moon reaches its culmination on the 16th Kalaa of Amaavasyaa (New Moon), marking the end of –
(1) A phase of Rotation of Day and Night – (End of Transitory Achievements of Worldly Life; End of Duality – New Beginning of Spiritual Life; New Beginning of Journey Beyond Kaal or Time)
(2) Body/Wealth – (End of Body or Physical Death; End of Material Existence, Arth, Power; End of Dharma-Artha-Kaama, that is beginning of Journey to Moksh)
(3) Power – (Power ends by Dand of some Superior Power; Moosal suggests Dand)
(4) Old – (a New Beginning starts from the 17th; End of Old Order of Power; End of Old Rulers; End of Old Self, that is Birth of New Self – both Spiritual and Material)

All these themes are evident in the 16th Parv or Mausal Parv, and in Krishn's apparent Self-destructive act.

As the 16th Parv, Mausal Parv represents "Body of Time" - Kaal Shareer (MBH, 12.309.24-25). It marks the end of the narrative of physical achievements of the heroes. Very significantly, soon after, Arjun suffers a humiliating defeat at the hand of mere Dasyu indicating the inevitability of "fall of pride and worldly achievement". Arjun too seems to be the victim of Internal-Matsyanyayam, the Big-Fish being Ahankaar in his case (MBH, 16.8.46-49). The Dasyu take advantage of Arjun’s Ahankaar.

It is natural then, that thereafter, Arjun receives a Kaal (Time) discourse from Vyaas – a timely discourse, so to say - to quit worldly ties for gatim mukhyam (MBH, 16.9.25-38).

That explains why Mausal Parv is not only called Ghoram (MBH, 1.2.68) and Daarunam (MBH, 1.2.220), but also Mausal Shruti Sankshepah Shisht Dwijanishevitah (MBH, 1.1.62 @ 1.54) i.e. it is the core of the Ved.

Mausal also means alligator in Sanskrit language, that is why krishn has been called "Dead Alligator" in Mausal Parv.

Krishn and No 8

Krishn Himself is associated with No 8 also--
(1) Krishn is the 8th Avataar
(2) Krishn is born on Ashtamee (8th Lunar Day)
(3) Krishn is Vasudev's 8th child
(4) Krishn is born in the 8th month of Devakee's conception (Harivansh)
(5) Krishn has 8 Chief wives (see Krishn's Wives)
(6) krishn had full control on all the 8 Siddhi.
(7)  According to a Tantra text Lord Krishn is the male form of goddess Kaalee, who in turn is the Lord of the Saturn, who is represented by the number 8 (in numerology) - Devee's day is also Ashtamee, 8th.
(8) The 7 notes of the scale (Swar), in Indian music are named as Shadaj, Rishabh, Gandhaar, Madhyam, Pancham, Dhaivat and Nishaad ...
Now, the last 7th Swar is "Nishaad" - Krishn the 8th dies at the hand of " (Nishaad is Jaraa is also Vyaadh = fisherman = Kaal-Kali-time) ...to become one with "1" ... superb

--"8" is a trans-cultural mystic number and is associated with Jesus also.
--The "8"th day in a Lunar Month is a day when the Moon is half-light and half-dark - indicating the White-Black paradox that Krishn represents.

Bheeshm - In Mahaabhaarat, the other "8" is Bheeshm.
--Bheeshm is the incarnation of the 8th Vasu. The name of the 8th Vasu is Prabhaas; and Yaadav are destroyed in Prabhaas Kshetra
--Bheeshm himself was 8th child of Shaantanu.
-- Bheeshm teaches Dharm to the Paandav with Krishn as the listener. It seems then, "8" represents Dharm.
I may conjecture that the two "8"s (8 + 8 = 16) speak (Bheeshm and Yudhishthir) what Mahaabhaarat stands for - Dharm."
--And why only half of 16, (16 is half of 32 = Anushtup Chhand - 32 =  8 x 4)

--Why 36 years of rule by Paandav merge with 16th Parv ("36" and "16" relation)
--Why Paandav leave at 17th Parv (Prajaapati = 17 fold)
--Why only 18 Parv (18 is half of 36)
--36 = 360 days = 36 syllables of Brihatee meter
--At end of 17th Parv, Yudhishthir calls Draupadee "Brihatee" - suggesting preparation for 18 (18 is half of 36) -
--Yaagyavalkya says Brihatee Chhand takes one to Heaven etc
--Mahaabhaarat of Anushtup meter must end at 18, because beyond it cannot take - when "Brihatee" Draupadee is in heaven, Yudhishthir wants to question her but ultimately cannot - AT and BEYOND 18 there cannot be 'word' - so, Mahabharata ends at 18.
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Regarding the Yin-Yang balance - yes, it is the Ardh-Naareeshwar that "8" represents because Shiv's association with "8" is very prominent in Anushaasan Parv.

"8" is undoubtedly the Ardh-Naareeshwar (Masculine-Feminine Balance) "FISHERMAN."

"In Mahaabhaarat, the other "8" is Bheeshm, and he teaches Dharm to Yudhishthir with Krishn as the listener. It seems then, "8" represents Dharm.

There is mention of one "8 x 8" "fight of meters" between Gods and Asur in Panchavinsh Braahman - in which the Gods emerge victorious when Prajaapati is invoked by both sides and he finally tilts to the Gods - a role very akin to Vyaas and Krishn. The Gods win over the Asur by using "8" meters against their "8" meters - that both sides did not have before Prajeapati's intervention. If Bheeshm fights for Duryodhan, he is not entirely on his side - and rather favors the Pasndav.

The 'fight of meters' is also in the imagery of phases of Moon.
In Mahaqbhaarat then, the Krishn-Bheeshm 'fight' or "8 x 8" 'fight' is actually a "8 + 8" (= 16) Dharm-Yuddh for Bhoo Bhaar Haran, and Bheeshm finally plays the "8" role in teaching Dharm to Yudhishthir and Paandav (and in Krishn and Vyaas' presence).
The Bhoo Bhaar Harana is accomplished with destruction of Yaadav - and significantly, the destruction takes place at 'Prabhaas' - and 'Prabhaas' is the name of the 8th Vasu, whose incarnate Bheeshm is!
It may be conjectured that the two "8"s (8 + 8 = 16) symbolically represent what Mahaabhaarat stands for - Dharm."
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Two other "13"-
(1) Bheem born on Trayodashee - daytime
(2) Duryodhan born on Trayodashee - night-time
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(1) In the cosmic scheme, even Vishnu’s power is not absolute.
Lord Krishn's death by Jaraa has connection with Lord Raam's boon to Angad that the latter will be able to avenge the death of his father in his (Lord Raam's) next birth. So Vishnu certainly liked to honor his own words.

Further Ved Vyaas Jee has said that Lord Krishn could have nullified the curse of Gaandhaaree. if He so wished, but He let it be like that. Vishnu is his own master at all times.
(2) Lord Krishn prayed to Lord Shiv to give him a son. The obvious meaning is that Lord Shiv himself should take birth as his (Lord Krishn's) son to destroy the invincible Yadu. Thus Saamb was born to Satyabhaamaa as the son of Lord Krishn. Thus it was Lord Shiv, who destroyed the Yadu. Lord Krishn did not want to appear as the killer of His own people (the Yadu). Of course, all beings are his own people.