How do you find a planets max population?

In my understanding there is no "max population".

Population increase is based on morale. If the people on that planet have good morale (over ~50) they will increase. If the people on that planet have bad morale (under ~40) they will decrease.

The planet class probably defines some sort of sweet spot that the population will tend to gravitate toward, however, morale is a very complex beast influenced by taxation, morale resources and most likely a number of other things.

The practical max at any given time is dertermined by your empires ability to absorb their low morale and when they stop reproducing because of morale. Building social projects will change this "ceiling."

~SDC~

And the more people you have, the more taxes you get - right?

Yes.

- Ash

~SDC~

and if your morale starts falling because it is overpopulated you can build a combat transport and ferry the surplus population to planets that might be underpopulated. And if you want you can freeze dry the population, send them to a far corner of the universe, where they will wait till you decide to invade those unlucky souls that live there

yours in TUNA
the cat

There is a max. you will not get more than 100 billion on any given planet. However only class 26+ planets will usually be able to achieve this anyway.

~SDC~