What’s the minimum invading arthe to leave in an occupied planet in Stellaris

You occupy the planet, you do not control it, this works similar to EU4.

The occupation grants a large amount of warscore and you can send your armies away, Your opponent has to reinvade to liberate the planet.

Whether you get the planet in the end depends on the war goals, the occupation merely provides additional warscore, with which you can press your demands.

Most importantly though, it prevents the enemy from using the planet and getting resources from it without you having to constantly bombard it and requiring an invasion from your opponent to liberate it.

So first you need to recruit the army on your planets. Then if you select that planet and go to the armies screen, in the top left of that panel there is a button named "embark all" this will move all your armies to the orbit of the planet where you can then move them like regular ships. Now move them to an enemy planet and you can click the button to land all of your armies. It's advised that you first bomb the planet to destroy fortifications and increase the chance your armies will be able to take over the planet.