[jeremyjh]

That isn't a bad idea but it really isn't an answer that helps us in the next several hundred years. A technological civilization that is fully self-sufficient would need millions of people in every profession like we have on earth. Vernor Vinge estimated 100 million people are required to run a civilization that could refit a starship.

[AstralStorm]

Sufficiently automated and focused, I'd drop one order of magnitude from it. (So 10 million, based on manufacturing, refining and mining estimates, assuming the materials are available etc.) Think of it like applying the city of Shenzhen to the task, if they wanted they could do it. Heck, build one from scratch if materials are there.

Having a (reasonably) sustainable 10 million sized habitat is quite hard.

[jeremyjh]

Shenzen doesn't sustain itself. A massive supply chain supports it with a population much larger than that of Shenzen behind the supply chain. Think not only of the farmers who grow the food, but also the supply chain of food production, the plants that process the fertilizer, all the components and raw materials of the factory machinery that processes grain, the transportation infrastructure that ties it all together and the supply chain of that transportation infrastructure. It just goes on and on and on.

[BurningFrog]

As people more technology makes people more productive, that number will shrink.

[jeremyjh]

This estimate was based on a projection of our technological state 10,000 years or more from now, but without super-intelligent AI or FTL travel. So human settlements are all over the galaxy, but they are fragile and disconnected from each other by hundreds and thousands of years of travel in slow time. One of the problems in this world is a civilization may have crumbled in the time it took to travel there and be in such a reduced state that its incapable of refitting your ship, leaving you marooned.

https://en.wikipedia.org/wiki/A_Deepness_in_the_Sky