Since you have more connections source then torrent, global limit is a thing. You have trackers and DHT. As I mentioned above, having uncontrolled connections can cause and network issues. TransmissionBT has global socket limit beside per torrent settings. One more issue about UPNP, UPNP port can be different from local port on busy networks where a lot of UPNP client concurrents for same "local port number", I guess current implementation is better to disable since is has a lot of major issues.