Oh, they mention that in Revenge of the Sith, I think. There can only be two Sith at a time, the Master, and the Apprentice. Then, once the apprentice has gained all the master's knowledge, he or she kills the master and they become the master, and they take on an apprentice. It works like that every time. See, that's where 'the Chosen One' comes in. Anakin did balance the Force. After he was done, there was two Jedi: Obi-Wan and Yoda. There was also two Sith: Palpatine and himself. Sorry, I'm a Star Wars nerd. :)