The maximum for a planet with an average tech level of 0 is 1 billion colonisits in the 0.30 game.

Then you add 250 million for each average tech level for the planet.

If your planets average tech level is 20 then..

1,000,000,000 + ( 20 * 250,000,000) = 6,000,000,000

So your level 20 planet can have 6 billion colonists on it. The most you could ever have on a planet is 14.5 billion colonists with a level 54 planet.