At the very end of Stat Wars episode 1 : The Phantom Menace, Mace Windu and Yoda had this discussion : 

> Mace Windu : There is no doubt. The mysterious warrior was a Sith.  
> Yoda : **Always two there are....no more...no less.** A master and an
apprentice.

Within the movie series, this principle is respected. The only exception I see is Grievous, but I don't think he counts as an apprentice only because he can wield some lightsabers, he does not appear to be a force user which is a major part of the apprentice description.

But in expanded universe, this rule seam the be broken fairly often. One notable example is [Asajj Ventress][1] from the Clone Wars animated series. How she (and others) could be a Sith if there is already two of them with Sidious and Dooku?


  [1]: http://en.wikipedia.org/wiki/Asajj_Ventress