You don't just need an ecosystem or contained biosphere. You also need an industrialized civilization that can fully support itself, producing everything used t

You don't just need an ecosystem or contained biosphere. You also need an industrialized civilization that can fully support itself, producing everything used to operate that ecosystem, and everything needed to send rockets into space - which is the very tip of an enormous technological pyramid built on the backs of many, many people. So you need an ecosystem big enough to support that, which might have a realistic lower bounded size of as many as a hundred million people. Perhaps a terraforming project could be accomplished in a few thousand years, but for the most part this is all ridiculous pipe dream. We only have one planet.