Most VPNs these days don’t allow for static ports; Creeps figured out how to use them for sharing CSAM. So instead, the VPN assigns you a random port every time you connect.
For your torrent client, it’s just a matter of going to your settings and updating the port number when you reconnect. If the system has a stable connection and doesn’t sleep (like a server) then you won’t need to do this often. The port only changes when the VPN reconnects, so it’ll only be an issue if you have a blackout or your internet drops.
As for torrenting without port forwarding, results will be mixed. Some trackers will still allow it, some won’t. Some will only allow very limited speeds. So on the two extreme ends, you may be able to torrent just fine, or you may stall even when there are hundreds of available seeds.