You only need to forward the ports which you configured in bittornado prefs and you only need to open as much ports as you want to run instances of bittornado. You also have to ensure that the ports are always forwarded to the correct IP, so if you don't have a static (LAN) IP and your IP changes you have to reconfigure the port forwarding to the new IP or it won't work, therefore a static IP would be a good thing.
Compare the WAN IP at your router status page with the IP shown on the
http://btfaq.com/natcheck.pl page. If they differ, you may be behind a proxy or your ISP blocks the ports, if not your port forwarding settings are probably not correct.