Mara was the Emperor’s Hand, which is NOT the same thing as a Sith apprentice. She was a tool, a Force-using James Bond brainwashed to be devoted to Palpatine, not a potential successor.
Maybe there could only be two “Darths.”
So what’s the deal with what’s-her-face from the Clone Wars cartoons? The girl with the two lightsabers? Did she count as an apprentice for the short time that she was around, or does that not count since she was just being used to get Anakin?
This is my understanding as well. There can be several ‘evil’ jedi out there using the dark side of the force, but only two (most often the two most powerful) call themselves the Sith.
I also seem to remember hearing/reading somewhere that originally the dark side was not evil. In fact, there were several Jedi, both light and dark side, doing what would basically be considered ‘good’ things. They were just two different ways of learning the force, really. Neither one better than the other, and neither one more ‘good’ or ‘evil.’ But at some point thousands of years in the past (by that I mean thousands of years before any of the movies) there was some kind of ‘falling out,’ and the dark and light side fought each other, and for whatever reason, the light side ‘won,’ and so pretty much the Jedi order vowed only to use the light side, so therefore everyone who used the dark side was against the order, and labeled as ‘evil’, whether they were or not. Eventually, truth caught up with the myth. So even as early on as events in Knight of the Old Republic (a video game series 4000 years before events in A New Hope) the dark side is labeled as evil, and indeed, evil acts is what brings characters to the dark side. It is also interesting to note that anyone can learn the dark side powers, even if not evil. It just costs more “force” to use them if you are light side, and they are less effective. And, in those series, there are dozens and dozens of dark Jedi, but still, only two Sith (and in the first one, you actually go to the Sith academy to try to become the new Sith Apprentice to either a) infiltrate and overthrow it if light side or b) actually be a sith if dark side.)
Papatine and Vader had many trainees, one of which might presumably have gotten a promotion in the event of a death somewhere. There were dark side prophets (who specialized in listening to the future), the Emperor’s Hand (of which there may have been several), minor dark Jedi and servitors, and odd experiments (such as Azrakel and Joruus C’Boath).
None of them came close to matching either Palpy or Vader, however, though a handful were arrogant or stupid enough to think so.