Concurrent active torrents? So you mean only the ones that has activity on them? Entirely depends on your system. If you have ssds, you can have a lot, with hdds, not so much. I would limit the upload slots per torrent to 3-5 and set the global upload slots to 75 or something like that. Looking at the statistics of the io time in queue can also help. I have about 19k torrents and 30-60 are active out of them on hdds. Once on EMP freeleech it hit 100+ active ones, that was when I limited it as that was too much.