Rule of Two (Earth-2356)

"Two there should be. No more, no less. One to embody power, the other to crave it."

- Darth Bane

The Rule of Two was a Sith philosophy mandating that only two Sith Lords could exist at any given time: a master to represent the power of the dark side of the Force, and an apprentice to train under the master and one day fulfill their role. The philosophy of two governed the Lords of the Sith for nine millenniums, beginning with its founder Darth Bane.

Source

 * https://starwars.fandom.com/wiki/Rule_of_Two.