QUESTION:
If we colonize Mars, how many people are going to be able to fit there?

ANSWER from Mark Adler:
Mars has about the same land area as Earth with a little less than
half the sunlight.  Unfortunately, that doesn't even come close to
answering your question.  The real answer depends on how those
people breathe, drink, and eat on Mars, and how much the natural
resources of Mars can deliver.  In short, I don't know.  How many
people fit on Earth?  We don't really know the answer to that
question either.  (We may find out the hard way.)