By the time we get to Mars we'll have pretty good robots. It's much easier to support them which is why they are trundling Mars as we speak and humans are decades off. It'd probably be a handful of people and a lot of robots.
At what level of industrialization? I find it hard to believe that, for example, one can build a 20nm chip fab with a million people, keep a fleet of emergency helicopters operational (you would need only a few, and that makes it harder to do) or launch a set of GPS satellites.
