I don’t know of any natural limitation to the number of tracks past your ability to see them all on the screen. I’ve run into that limitation multiple times and the show survived.
The total number of tracks in a single Audacity Project is literally limited only by the total number that can be counted using a signed 32-bit integer – a ridiculously large number; however, each track (even when empty) gobbles up RAM and video resources so the practical number is governed by those two resources.
The total number of Projects, regardless of the number of tracks each contains, is limited. The actual limit is dependent on operating system and to a lesser extent on hardware; Linux and Mac OS operating on fairly modern hardware is limited to about 90 open projects at a time; Windows XP/Vista/7/8 is much more severely limited – again on fairly modern hardware, this value is somewhere in the upper 20s to lower 30s.