Well, in the planet entity file, you can only have 5 "meshes"....by meshes I'm simply talking about those first blocks at the beginning of the file where you set the mesh, icons, and string to be used...you'll notice that planets in vanilla don't actually have multiple meshes, but may have different icons, names, and types of dust or asteroids (asteroid belt is a great example of this)...
That being said, if you had 15 different textures that all represent "terran" planets, you could just have 3 planet entity files (terran1, terran2, etc.) where each had 5 different meshes...in your galaxy scenario definition, each of those 3 entities would be listed as orbitBodyTypes, but you could then have a planetType (call it Terran) that lists all three different types of terran planets...whenever you make a map, you'd refer to the terran planetType, which would randomly select from each of the 3 entities (and from there one of the 5 random meshes would be chosen)...
As far as number of planets, I don't think there is a hard limit...however, this game cannot utilize more than 2 GB of memory, so be careful having too many textures...you can save a lot of space by compressing them, but still you will be limited by how many planets you can have simply by this restriction...
