Assuming population able to be centered at some location, how many people are needed to maintain current level of civilization and progress.
I wonder if it isn't impossible to do because so many things are distributed, even if you were centered at point X well the necessary minerals to maintain specifically technologies are all at point Y so that means you need to have distributed population, but that means necessary redundancy of services like health etc. distributed.
In theory, humans can rediscover that knowledge in a few centuries/millenniums. However, it's also possible that we'll never regain that knowledge since we've already exhausted natural resources on the surface. Without easy access to iron/coal/oil, we wouldn't be able to create machines that we have today to extract resources deep underground.