Star Wars has retconned the Sith's Rule of Two, which had been the way the dark side order operated for a millennia. The Sith emerged from the shadows in Star Wars: Episode I - The Phantom Menace, when Darth Maul killed Jedi Master Qui-Gon Jinn before himself being defeated by Obi-Wan Kenobi. "Always two there are," Yoda observed ominously at Qui-Gon's funeral. "No more... no less. A master and an apprentice."
The prequel trilogy followed this format to the letter. Darth Sidious was the Master, and he went through apprentices at a quite remarkable rate; he seems to have already been training Dooku before Maul was believed killed, but even the Count of Serenno was little more than a place-filler. Palpatine had set his eyes upon Anakin Skywalker, the Chosen One, and he gladly sacrificed Dooku in his successful attempt to seduce Anakin to the dark side of the Force.
But the Star Wars sequel trilogy has not followed that formula. Star Wars: The Rise of Skywalker revealed the Sith redoubt of Exegol, a legendary world hidden in the depths of the Unknown Regions. There, Sith cultists flourished under the rule of a resurrected Darth Sidious. The recently-released junior novelization of Star Wars: The Rise of Skywalker attempts to resolve this issue with a smart retcon; it turns out the Jedi had misinterpreted the Rule of Two, as Rey discovers when she reads an ancient text contrasting the Sith and the Jedi.
"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."
In the prequel trilogy, the Jedi believed the Sith operated under the Rule of Two; that there were only ever two Sith, a Master and an Apprentice. No doubt Darth Sidious was keen to encourage this error, keeping the existence of Exegol a closely guarded secret. In reality, the Sith are Ruled by Two. The Sith Lord sits upon the ancient throne at the heart of Exegol, with an apprentice who will one day take their place - unless, like Palpatine, said Sith Lord masters the power of Essence Transfer.
This is a smart retcon; it preserves the heart of the Rule of Two while neatly explaining the existence of an entire army of Sith cultists and legions of Sith Stormtroopers. It even fits with the fact Darths Sidious and Vader were together referred to as the "Lords of the Sith" - because that, quite literally, is what they were. Presumably the Sith cultists were encouraged to compete for the attention of the Lords of the Sith, and sometimes a Sith Lord would choose an apprentice from among their number if they proved particularly strong in the Force.
That would also explain another oddity in the history of Star Wars; why the Jedi never discovered the continued existence of the Sith, when these sinister beings were involved in galactic affairs. If there were indeed times the Jedi came close to learning the truth, the Sith could retreat back to Exegol for a time, and a Sith Master could even choose a local resident as an apprentice rather than comb the galaxy for someone suitable. Thus the Sith remained in the shadows, biding their time - until Palpatine.