::"Mega-City One, a megalopolis of over 400 million people across the east coast of the United States, features in the Judge Dredd comic, serialised in 2000 AD."
From "megacity" on wikipedia.
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Assuming a population growth rate of 2%, post-Apocalypse, you've got 400 million, with 8 million being added each year from then until Necropolis.
That's approx. 400 + 64 = 464.
Necropolis then kills off 60 million, so we're back to 400-ish, and assumed growth up to this year would be:
400 + 136 = 536 million
However, as was said, there's Judgement Day - there must have been a high death toll, there - Total War nuked out a few sectors. Did Sin City's infection spread to the city? I thought it was contained?
We're looking for about 100 million deaths to get it to 400, 420.