In the Phantom Menace, we learn about the Sith Rule Of Two.
Always two there are, no more, no less.
This has been further expanded in various canon and non-canon works.
But in Star Wars Rebels we meet the Inquisitors: Force-wielding beings with lightsabers who work for the Emperor.
Although there are plenty of Force users around, how do these Dark Side people fit in?
Presumably can't be in the Sith order, otherwise that would break the Rule of Two. Or is the Emperor just playing semantics?
The use of dark side allies such as the Inquisitors and the Nightsisters was not a violation of the Rule of Two since they were not considered Sith. The distinction between a Sith and a mere dark side ally is important because the Sith had secrets which they did not share with other dark side users. For example, there was a Sith shrine below the Jedi Temple which "even the most powerful of Dark Side Adepts" did not know about:
The two of them were in Sidious’s lair, a small rock-walled enclosure beneath the deepest of the Palace’s several sublevels that had once been an ancient Sith shrine. That the Jedi had raised their Temple over the shrine had for a thousand years been one of the most closely guarded secrets of those Sith Lords who had perpetuated and implemented the revenge strategy of the Jedi Order’s founders. Even the most powerful of Dark Side Adepts believed that shrines of the sort existed only on Sith worlds remote from Coruscant, and even the most powerful of the Jedi believed that the power inherent in the shrine had been neutralized and successfully capped. In truth, that power had seeped upward and outward since its entombment, infiltrating the hallways and rooms above, and weakening the Jedi Order much as the Sith Masters themselves had secretly infiltrated the corridors of political power and toppled the Republic.
Save for Sidious, no sentient being in close to five thousand years had set foot in the shrine. The room’s excavation and restoration had been carried out by machines under the supervision of 11-4D. Even Vader was unaware of the shrine’s existence. But it was here that they would one day work together the way Sidious and Plagueis had to coax from the dark side its final secrets. In the intervening years he had actually come to appreciate Plagueis for the planner and prophet he had been. Such perilous machinations required two Sith, one to serve as bait for the dark side, the other to be the vessel. Success would grant them the power to harness the full powers of the dark side, and allow them to rule for ten thousand years.
Tarkin, p. 101
There were still only two Sith -- Sidious and Vader -- so the Rule of Two was followed.
That said, the use of dark side allies could be considered a violation of the spirit of the Rule of Two. The Rule of Two required the Sith apprentice to defeat the Master in a one on one duel to ensure that each new Sith Master was stronger than his predecessor. The Rule of Two also prevented the Sith from fighting each other for greater power instead of their light side enemies (though this was less of a problem for Sidious and Vader with the Jedi Order almost entirely eradicated). Without the Rule of Two, a group of Sith apprentices could band together and overthrow the Sith Master by sheer numbers -- if that happened, the new Sith Master would be weaker than his predecessor. By using dark side allies such as the Inquisitors, there was theoretically a danger that Vader and the Inquisitors could band together and defeat Sidious -- Vader would become a new Sith Master but would have been weaker than Sidious. This would have been an unlikely event, though, and in the end that particular threat did not materialize.
The Inquisitors are not Sith. They are only dark side Force users. Palpatine lured them to the dark side, but only to hunt down remaining Iedi. In the Darth Vader comic Palpatine purposely lets Vader and the Grand Inquisitor fight, but Vader defeated him easily. Afterwards Vader asks Palpatine the question that if the Inquisitor won, would he be standing by his side. That proves that even though the Emperor used them for Sith matters it's still only Palpatine and Vader, master and apprentice who are real Sith. I believe there probably would have been other dark side Force users in the past since anyone who is strong with the Force can use it for good or bad, it doesn't necessarily make them Sith.
My thoughts are that the Rule of Two was dreamt up by Darth Revan to make the Sith Order stronger. Darth Bane implemented this rule thousands of years later by killing all the Sith. But the purpose of the Rule of Two is to get stronger in secret until the Sith could finally conquer the Jedi. So my guess is, since Palpatine did just that, the Rule of Two is no longer needed. But because he is careful, he keeps them as just dark Force users instead of giving them the title of Sith.
