Darth Vader was the Dark Lord of the Sith. Under him were six Knights of the Sith. Why only six? Palps probably put a cap on how many Sith Vader could train, just to fuck Vader over. His First Knight was Jerec from the Dark Forces books/games. After Vader kicked the bucket, Jerec became the new Dark Lord. What became of the Second-through-Sixth Knights, I can’t say. Maybe they all died/defected/didn’t get along with Jerec. Whatever the case, he was gonna continue the “Rule of Seven” with knights of his own choosing. Then Kyle Katarn kicked all their asses.
This pertains to the pre-1999 SW Universe, BTW.