I wouldn't put half of humanity anywhere right now. You just need enough to keep a population (with immigration) after they face a horrific mortality rate as they figure out how to lower the risk; hopefully highly trained and well equipped as sparse mitigation, but exploration and pioneering are hungry gods that will be fed human sacrifices. I'm personally more partial to an O'Neill cylinder or a Lunar colony than Mars and the lunar colony will probably come first, Mars is a big step backwards in delta-v, but yes the mass required to get that going is currently prohibitively expensive, and the biosciences are going to be really tricky.