Originally, there were only two Sith: one master, one apprentice. Then it occurred to writers that in order to keep those numbers steady, there would need to be lots of backups: if an Apprentice turns on his Master and is killed, the master will need a new underling to slot into the hierarchy; that's how Darth Sidious had Darth Tyranus ready to go as soon as Darth Maul got cut in half, and then immediately started grooming Anakin to take over from Dooku. On the other hand, if you successfully overthrow your old boss and become the new Master, you'll need someone to promote right away; that's why Vader was training Galen Marek, Dooku had Asajj Ventress... heck, even Palpatine was already raising Maul before Darth Plagueis had died!
Anyway, having potential apprentices requires a pool of Force-sensitives to draw from, and Palpatine knew if he could control that pool, it would 1) keep Vader from plotting against him, and 2) give him plenty of spares in case "Robot-Lungs Skywalker" should go out of warranty. Thus the 1987 Star Wars Sourcebook roleplaying supplement introduced the concept of the Inquisatorius, a group of dark side adepts who worked for the Emperor as part of the Intelligence division. The concept was reintroduced to the new continuity in Star Wars: Rebels and expanded upon in Jedi: Fallen Order. The group is not trained in the way of the Sith, and their constant in-fighting and jockeying for favor keeps them from posing a threat to either Palpatine or Vader.
The members include:
- The Grand Inquisitor
- The Second Sister (Trilla Suduri)
- The Third Sister (Reva Sevander)
- The Fourth Sister
- The Fifth Brother
- The Sixth Brother (Bil Valen)
- The Seventh Sister
- The Eighth Brother
- The Ninth Sister (Masana Tide)
- The Tenth Brother (Prosset Dibs)
Come on, Hasbro, make a full collection!