In the Star Wars universe, there lies a fundamental principle known as the “Rule of Two” among the Sith. This ideology states that there should only be two living Sith at a time; a Sith master to embody power, and a Sith apprentice to crave it. This practice originates from the Sith Lord Darth Bane.
Darth Bane is a Sith Lord character originally appearing as part of the Star Wars expanded universe. In the second novel written by Drew Karpyshyn, entitled Star Wars: Darth Bane: Rule of Two, Darth Bane first indoctrinates this Sith rule.
When a Sith apprentice has reached their full potential in the ways of the dark side, the apprentice generally takes the life of their master causing transference of power from the master to the apprentice. The new Sith Lord Master then selects their apprentice and thus the cycle of the Rule of Two repeats.
Darth Bane believes that one true, all-powerful lord of the dark side should exist among the Sith. So he destroys the remaining Sith with the dark side of the force using a technique known as the thought bomb. Barely escaping the destruction himself, Darth Bane, and his 10-year-old, female apprentice named Zannah, then go into hiding from the Jedi.
The practice of the Rule of Two continues for almost 1,000 years until the Sith Lord Darth Tenebrous decides to break it by claiming two apprentices in an effort to exact their revenge on the despised Jedi Order.