The engine probably doesn't care how many planets there are in the game, but your RAM certainly will. You can easily go above the 100 planets/four solar systems, but much farther and your rig will probably break under the 2GB RAM limit. A guy who knows his computers supposedly was able to use 12GB on a Linux, in which case that might handle a thousand, but for must of us our Rig won't go anywhere near that high.

There are a couple of ways to get around this though, like the Sins optimization mod, to push your limit a bit farther.

I read on an old post somewhere that the max was 512 stars and 512 planets on map making but good luck running that.

Random Multi-Star Huge is 5 Stars and I never counted but a lot of planets (maybe 50) at each star. This will start to lag later in the game even with TSOP mod running.

EDIT: PDFs only as far as I know. Start with the SoaSE Wiki if you want a head start.

I'm running a 10 player 399 planet 1 star game. You really don't need it that big though. It takes way too long.