There are
4,416 cities in world with population of at least
150,000 People
So by that account we are looking at anything between 4,416 to 5,000 Nuclear weapons of a triditional ww2 style weapon
Modern cities do also have radius over 30km so chances are there would be some survivors in most cities , I would imagnie the post bomb nuclear radiation impact would kill more.
For each city may be 1% or 5% would survive
Then likely it will be a Hunt for food / water
But we (Humans) do have some advance grade weapons , which send dust into Atmosphere , and when that happens likely looking at 1-2 years of cold freeze weather. Not sure exactly how much power in weapon results in that kind of devestation
But what ever we Humans Make , a meteor strike will be Million times stronger
Pretty much 0% chance of survival when that happens if not for the heat wave the water wave will do it
Quite nasty these Gigantic waves
Destructive forces in Nature > Any thing Humanity can even imagine