First the reference from the scripture:
"Gandhari said ... Since deliberately, O slayer of Madhu, thou wert
indifferent to this universal carnage, therefore, O mighty-armed one,
thou shouldst reap the fruit of this act. By the little merit I have
acquired through waiting dutifully on my husband, by that merit so
difficult to attain, I shall curse thee, O wielder of the discus and
the mace! Since thou wert indifferent to the Kurus and the Pandavas
whilst they slew each other, therefore, O Govinda, thou shalt be the
slayer of thy own kinsmen! In the thirty-sixth year from this, O
slayer of Madhu, thou shalt, after causing the slaughter of thy
kinsmen and friends and sons, perish by disgusting means in the
wilderness. The ladies of thy race, deprived of sons, kinsmen, and
friends, shall weep and cry even as these ladies of the Bharata
race!’"
Now to understand the magic number, first we have to rationally understand what is a "curse" (or a "blessing")?
The curse is not something regular "bad-mouthing" or "trash-talking" we see in today's society or media where people give to each other. That is just a frustration or anger of short sighted people.
A Curse (Shraap) or a Blessing (Aashirwad) in its true sense is a "definite Prophecy".
No one can change the course of events of past, present, future except God. Since God has decided everything, he need not change it. Here is a nice Q&A on the same topic.
Is our destiny pre determined? If yes, then why do our actions affect our karma?
Such "spiritual capability" of issuing genuine curse (or blessing) can come by various means:
- Having a strong knowledge of subtle activities of Karma through
experience or enlightenment
- Having gone through sever austerities and observing own Dharma,
with utmost perfection
- Being in true feat of rage or happiness, certain doors of mind are
opened for a short while which helps predicting future (relevant today)
- Natural power acquired from the deeds of past births
- ...
We may count Gandhari in the 2nd league. She had observed her Stree Dharma with utmost perfection throughout her married life and hence she had certain blessings from various elders like Bhisma, Vyasa. Refer this excerpt:
Devoted to her lord, that highly blessed lady had always practised
high vows. Undergoing the severest penances, she was always truthful
in her speech. In consequence of the gift of the boon by the great
rishi Vyasa of sanctified deeds, she became possessed of spiritual
knowledge and power.
Gandhari was well aware that Duryodhana was foolish and never obedient to elders. The Paandavas were so much essential to be saved & unharmed. But as an emotional mother of all the slain sons, she also was in feat of rage towards Vaasudev Krishna, who in her opinion could have saved this war.
Due to increasing power, Yaadavas too were on the same path of self-destruction like Kurus and other kings were during Mahabharata war.
With her spiritual power of foreseeing the future along with anger of a mother she predicted something which was anyway going to happen in 36 years.
Ofcourse, Krishna too wanted it to happen and he accepted it whole-heartedly. Though Gandhari cursed for Krishna being the reason of killing his kinsmen, Krishna actually was not the direct reason for that. Instead he chosen them to kill each other. Now we can interpret it to be "alteration" or "correction", it's up to us. In nutshell, "36 years" was not a "chosen" figure, rather an already "destined" figure.
Hearing these words, the high-souled Vasudeva, addressing the
venerable Gandhari, said unto her these words, with a faint smile,
‘There is none in the world, save myself, that is capable of
exterminating the Vrishnis. I know this well. I am endeavouring to
bring it about. In uttering this curse, O thou of excellent vows, thou
hast aided me in the accomplishment of that task. The Vrishnis are
incapable of being slain by others, be they human beings or gods or
Danavas. The Yadavas, therefore shall fall by one another’s hand.’
Gandhari could have cursed (at least 4) Paandavas about the way of their to-be deaths if she wished, but to save her from doing that, here is the advice of Sage Vyasa to prevent that complete hopelessness for "to be rulers":
The rishi, addressing his daughter-in-law at the proper moment, said,
‘Do not avail thyself of this opportunity for denouncing a curse. On
the other hand, utilize it for showing thy forgiveness. Thou shouldst
not be angry with the Pandavas, O Gandhari! Set thy heart on peace.
Restrain the words that are about to fall from thy lips. Listen to my
advice. Thy son, desirous of victory, had besought thee every day for
the eighteen days that battle lasted, saying, "O mother, bless me who
am fighting with my foes." Implored every day in these words by thy
son desirous of victory, the answer thou always gavest him was,
"Thither is victory where righteousness is!" I do not, O Gandhari,
remember that any words spoken by thee have become false. Those words,
therefore, that thou, implored by Duryodhana, saidst unto him, could
not be false.