So I was actually poking around in the MapGenerator code, and I note that each IMapGenerator takes a "numberToSeed", which seems to be the number of planets. However, we don't have a means of controlling that from the start of the game, which makes it hard to experiment properly.
I would add a drop down menu or something to Window_GameSetup.cs for my testing purposes, but I don't see exactly how to pass that information along to the map generator. My guess is that you should be able to get that information into a WorldSetup object, but my attempts to disassemble ArcenAIW2Core.dll to see check the variable for that have failed. If I asked nicely, could you tell me the name of the value in WorldSetup to set to allow me to tweak the number of planets? Or is that not implemented yet.