Answer:
There are always only three true Sith. There is the Sith Lord, and the Sith Lord's apprentice. The apprentice secretly trains his own apprentice, and then the two overthrow and kill the Master. The greater apprentice takes up the mantle of Sith Lord, and the lesser apprentice takes his own secret apprentice. This cycle continues eternally, and creates a Darwinian Survival of the Fittest environment, violently forcing the Sith to become stronger and stronger.