The default is 6346 but you can choose whichever ones you desire so long as they are not officially used by other programs. Good alternatives are ports 49152 - 65535. But whatever you use in your router, you need to also make that setting in LW. LW's menu bar, Tools>Options>Advanced>Firewalls.

Oh & if you are port forwarding, only one computer can use that port number at a time.