Each solar system contains 15 slots for planets, plus one uncolonizable slot, Outer Space. Each planet may have a debris field and/or a moon. This allows for a total of 67,365 possible planet locations.
Each galaxy is a distance of 20,000 units from the next and previous numbered galaxies.
In order to get an overview of the whole systems and the player's locations, the community founded user-projects like ogame.gamestats.org, where everybody can help to create a huge map.