I know that the limit in C2 is 60 but what about this game?
C2 has no real addon car limit, Mastro's Ultimate Car pack has 1600+. We used to say that it depends on your computer's specs but we don't really know what's going on. I remember an old installation of C2 going funky once I had like 120+ addon cars.
I recently hit a limit with C1 but I can't remember precisely, it might be around 129 or 130 (I mean, total installed cars).
As I tested - for single player you may put about 126 cars max, but for net play it crash when there is more than 85 cars.
Also for network play you need to extend caricons.pix (modified version is included with MeldPack, Mastro's Special Edition and UCP).
If you want to replace all original cars you also need to take care to have proper amount of each car type.
I mean "hawk" and "eagle" (6 or more of both), "all" ("non cheat" - I think at least 6, but not tested). Also amount of cars with same "Strength rating" level is important, game must have possibility to took cars to play form each "SR" level. Look results of this test - http://qtz.toshiba-3.com/tests/strength_rating_test.zip