Star Wars has retconned the Sith's Rule of Two, which had been the way the dark side order operated for a millennia. The Sith emerged from the shadows in Star Wars: Episode I - The Phantom Menace, when Darth Maul killed Jedi Master Qui-Gon Jinn before himself being defeated by Obi-Wan Kenobi. "Always two there are," Yoda observed ominously at Qui-Gon's funeral. "No more... no less. A master and an apprentice."
The prequel trilogy followed this format to the letter. Darth Sidious was the Master, and he went through apprentices at a quite remarkable rate; he seems to have already been training Dooku before Maul was believed killed, but even the Count of Serenno was little more than a place-filler. Palpatine had set his eyes upon Anakin Skywalker, the Chosen One, and he gladly sacrificed Dooku in his successful attempt to seduce Anakin to the dark side of the Force.