One of the torrent file spec descriptions is over here.
The file format itself doesn't really have limitations like you're asking about. There might be one related to hash collisions but those are pretty unlikely to occur naturally.
Torrent utilities themselves may impose limits. The main one is, while integers in the torrent files have no upper bound, software may impose a 64 bit limit making the upper bound 2^64-1 at most. Given that this number is far larger than the storage capacity available to most people, it's unlikely you'll hit it. (To quote HG2G: "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.")
Next are practical limitations: Your available memory and storage capacity will impose the usual limits.