I forgot who said it but someone in this site wrote about this in a post and said that about 1 billion people would die due to the nuclear Winter and aftermath
They're an idiot. I even wouldn't put deaths due to the cold and failed crops after an all out Nato/USSR exchange at that amount.
India comes out on top. According to wikipedia, India has 110 nukes to 120 for Pakistan (none active on either side ATM), but at 1 bomb per city (and assuming that bomb wipes out the cite, irrespective of the physics), the world population review site puts it that Pakistan's largest remaining city will be Sibi, with 64,069 people, while India's largest will be Muzaffarpur, with more than five times that (it's among 9 that reach that mark). Interesting to note, the total loss of those cities would see Pakistan's population drop by more than 23%, while India's would drop by less than 11%.
They wouldn't drop one bomb per city and on average the weapons will be small (probably less that 10kt) leaving a good chunk of the cities still in tact.
If they have any sense the weapons will be directed towards destroying the ability for their enemies to wage war i.e. enemy nuclear weapon delivery systems, military HQs and large military bases.
But yes, India will most likely come out on top. Their GDP trumps Pakistan's enabling them to rebuild faster.