Well, somewhere there have to be a limitation of numbers of stars and planets - basically the game needs to loop through the stars and planets until the images have been loaded and the names read, and I suppose the limit is if you fill the entire screen with stars or planets so there's simply no room for any new ones. It's an infinite loop that stops when there are no more stars or planets to process, so there should be no maximum number aside from the size of a long integer (4,294,967,295).
Rest assure that you will never, ever, run into a limitation. Oh, and did I mention that if you have Multimedia Fusion 2, you can reprogram the starmap into anything you want.