You will want to use a port number above 10000 to avoid being blocked or blacklisted. Many experts recommend a port in the ephemeral range of 49152 to 65535. The default port used by many torrent clients is 6881 to 6889.
In reality, your port number doesn’t matter too much, so long as it’s not being used by any other apps or services on your device.
Note that most VPNs don’t offer port forwarding for security reasons. PrivateVPN is one exception that allows you to set up port forwarding in the VPN app.