The Sith are the ultimate villains of the Star Wars saga, though they have often felt like a rare sight. By the time the Skywalker Saga begins, the Sith have been in hiding and believed extinct for a thousand years. Not only have the Sith remained hidden from view, but the Rule of Two has reduced them in number, meaning only two Sith Lords have existed in the galaxy at any one time.
Source：https://www.cbr.com/star-wars-canon-sith-lords-outside-skywalker-saga/
