- Count the numbers of dead in the original two atom bombs;
- Count the number of devices reported to be in Pakistani inventory;
- Multiply the two, #1 by #2;
- Do you seriously advocate mutually assured destruction? Every single study so far has suggested that any exchange of nuclear weapons will lead to a nuclear winter, and that millions more will die during the aftermath, due to injuries received, or due to illnesses contracted due to exposure to radiation during the bombings.
I have done something better. I have taken the data from nukemap (basically population density data) and made a simple simulation to find maximum number of deaths for 200 20KT bombs -- which is the maximum yield Pakistan has. I consider both injury and deaths as deaths. The number comes much less than 400 million. This will still leave 1 billion indians alive.
I cannot say the same thing about Pakistan, however. Even with 20-30KT bombs that India has, it can kill entire population of Pakistan if it wants. Actually, Indian can easily win the war by causing a mass carnage in Punjab, rest will capitulate very fast. There is a learning from your 1965 war... You need to know where to hit.
BTW, this assumes that Pakistan will be able to deliver EVERY SINGLE of the weapons, every single one of the is 20KT (Pakistan has much more number of less than a kiltonne bombs because it wants to fight a nuclear war unlike India which wants to create a deterrence) and every single one of their weapon works. This is the limit of deaths they can cause.
In a nuclear war, India will survive... badly badly hurt though.
Now, lets take what you have suggested : 20KT nuke of nagasaki caused 80K deaths (
https://en.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki)... Lets double it. Make it 160K. So how many will 200 bombs kill? 200 * 160K = 32 million. Remember, India lost 4-10 millions in covid pandemic and though terrible loss, it did not lead to end of India. Admittedly, nuclear war will cause worse deaths, Indians will survive. Pakistan does not have enough firepower.