I'm thinking in a new DAPP that may use whisper extensively. To evaluate the viability of the DAPP, I need to know if there is a limit on the number of topics that a whisper message can have. I would like also to know how many topics a single filter can listen to, and how many whisper filters can coexist in the same node.
At this point it's difficult to give a very accurate answer to your questions as Whisper is still very much in its infancy stage, however I'll attempt to answer your question with the implemenation that's currently in operation.
The amount of filters you can use is not specced, it depends fully on the implementation.