Are large populations needed to make something like SpaceX a viable option ?

Large populations of gullible people with nothing better to do with their lives make it viable.

Yes. They will take lots of people into space, just like the airlines. 

Musk, who is now self-investing, is planning to send a million people over by 2050. I suspect mostly hookers and cheap prison laborers to the mines and the Love City.

SpaceX is a very viable business. If you mean the  colonization of Mars project by Elon Musk, that died over a year ago. The investors in that project pulled out in 2018.