The canon of that galaxy far, far away established a long time ago that as opposed to the Jedi, there are only ever two Sith Lords; a master and an apprentice. Except, the new junior novelization of Star Wars: The Rise of Skywalker retcons this and explains what the rule really implies.
After the fall of the Sith Empire, Darth Bane created the Rule of Two; that there should only be a Sith Master and a Sith Apprentice at any given time. These dark-side practitioners followed this rule for millennia, and even when they emerged from the shadows in The Phantom Menace, Yoda reflected on this during Qui-Gon’s funeral. “Always two there are,” he ominously said. “No more… no less. A master and an apprentice.”
The last movie in the Skywalker Saga, though, wasn’t exactly clear on how so many Sith cultists, some of them probably even Force-sensitive, served Darth Sidious on Exegol without breaking the rule. If you remember from the Prequel Trilogy, even Palpatine zealously adhered to this rule and went through apprentices in each of the three installments.
Well, the new junior novelization for The Rise of Skywalker resolves this issue via a smart retcon, by revealing that the Jedi misinterpreted the Rule of Two by taking it literally.
Going through an ancient text as part of her training, Rey comes upon a piece of writing that draws a clear contrast between the Sith and the Jedi and their influence on the galaxy. As the novel reads:
“The Prime is one, but the Jedi are many. The Sith were Many but often emerge Ruled by Two. The Seeds of the Jedi have been Sown throughout the Galaxy, on Ossus, Jedha, Xenxiar, and Others. The Sith have no Seeds, since what they Bury does not Grow. They are the Despoilers of Worlds, and have Laid to Waste once Fertile Habitats such as Korriban, Ziost, Ixigul, Asog, and Others.”
The Jedi Council from the Prequel era believed that the Sith operated as two. Palpatine may have been keen to encourage this error, not wanting to out his secret hideout on Exegol. But apparently, the Sith are not two and instead, actually ruled by two, a master and an apprentice, hence the name of the principle.
Of course, this is a pretty interesting retcon, adding a new depth to the established Rule of Two while also preserving its general meaning. Although, it isn’t exactly the first time that Star Wars: The Rise of Skywalker changes something fundamental about our understanding of this fictional world, and we have a feeling that it won’t be the last.