I have added about 40 ships to the game with my TNZ mod, and the game loads noticably slower. I would say there probably is an upper limit, based on your PC performance. If you play online, that limit is a lot lower because everyone has to be able to load the game. Just judging by what I've seen so far, I'd say adding over 100 ships might make the game load so slow it crashes. I'm going to find out though, because in patch 1.2 of the TNZ mod I add another 40.