[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.