Most dense population areas do not exist in a bubble.

This question came up in my mind when I considered, if you wanted to hypothetically put communists in a camp, let them run it, and then claim they starved themselves when they inevitably do, how much land do you have to give them to claim that they are responsible for starving themselves.

But that digresses. What do you think is the theoretical limit of how dense a population could get in a closed system? Clearly NYC couldn't support itself.