There are no limits in the sense that you're used to. It has the entire US & Canada RadioReference DB on board, so if the system is in the DB, it's within the radio's "limits". Take a look at something stupidly huge like Michigan's statewide trunk system, for example. IIRC that has several thousand TGs.
Like wise871 says, there is a limit on the number of favorites lists you can have, and also, each favorites list is limited to 1 MB in size. Realistically, you shouldn't be hitting either of those limits, because favorites lists can have a nearly infinite number of systems within them, so there's no need to have a huge number of favorites lists. As for the size, the entire CAN&US DB takes up around 40 MB of space on the SD card. The largest file for a single state (the whole thing, including all trunk systems, all county freqs, etc etc etc) is a little over 800 KB, or 0.8 MB. So you'd have to duplicate more than an entire state of data to overload a favorites list.