The Sith were a secretive order devoted to mastering the dark side of the force. Sith culture was influenced by the centuries of conflict between the dark side worshiping order and the Jedi, the defenders of the Republic.
During the Clone Wars, there were known to be only two, a master and an apprentice, a practice created after the New Sith War by Darth Bane. The Sith Order, like its Jedi counterpart, had hundreds of thousands of members. However, after the devastation visited upon the order by Republic forces, Darth Bane, one of the last survivors of this period, devised this Rule of Two.
The Rule stated that there could be no more than two true members of the Sith Order the master and the apprentice, no more no less, this doctrine allowed the order to strengthen and hide Republic for centuries until their existence was revealed during the Invasion of Naboo.