you would also have 2 figure out how the occupied persons planet can be un occupied??
Simple they have to rebell aka kill 25-50% of the attackers "forces/supplies" thus the attacker/occupation can't go on and the planet is freed,
To answer about a "maxium" limit yes we'll need one as it will prevent larger alliances/players from grabbing planets and farming them for profits while the "owner" can't do much to stop them... Also we all know occupations are not permit and over a x amont of time the occupation forces will retreat or become demoralized or run out of supplies thus can't occpy and will be killed by the owners colonist forces which means the occupation force will need supplies or they will automatic "Die" off from rebelling attacks. Also the occuaption force can only be x% of the total population (but can't be more then 75% that will prevent people from sending in forces that is 10x larger and preventing people from rebelling...
Edit: Changed 65% to x% and added a little bit...