At the very end of Star 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 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?