I think their official policy is unlimited SWA and up to two offline jumpseaters. I have seen them take more offliners though. The number for the automated listing system is 866-SWA-PLAY.
On a personal note I think SWA is the best airline to jumpseat on. Almost all of the stations pre-board you if the flight is not sold out. Everyone I have run into - gate agents, FAs and pilots have treated me great. And I have had gate agents and pilots really work some magic at times to get me on a flight. I once got on a plane that was oversold by 30. Somehow the pilots and gate agents worked out something that got me on. Anyway to any SWA folks who are here, thanks a million for all of your help.