It seems that max population for a colony is 5m times terraform level. So Terran planets, with terraform level 80, have a max population of 400m. A lava planet has a base terraform level of zero so max population would also be zero. I suspect you would need to terraform a lava planet before it could sustain any population. I'm interested to know. If you go to your planet list, does the planet appear in your colonies tab or your outposts tab?