While Starkiller was eventually killed, he used the Dark Side of the Force and could be considered a member of the Sith Order. In the game, which was canon at the time of release, Vader trains his own apprentice, Starkiller. Another example comes from Star Wars: The Force Unleashed. That would mean that throughout the events prior to Maul's death, there were three active Sith. For example, it's revealed in Star Wars: The Clone Wars that Maul survived the battle on Naboo, and lives on throughout the events of the first three movies, only dying briefly before the events of A New Hope. So while the rule seems to hold up, it breaks down the second you move beyond the main Star Wars canon and into supplementary materials.
RELATED: Rise of Skywalker Novelization Confirms Is to Blame for Kylo Ren Instead, Vader turns on his master, and while Vader is killed, Palpatine survives in secret, taking on a new apprentice: Kylo Ren. Fast-forward 19 years to the original Star Wars trilogy, and Palpatine attempts to lure Luke Skywalker to the Dark Side of the Force, forcing him and Vader to duel. After Dooku is later killed by Anakin, the fallen Jedi Knight goes on to become Sidious's new apprentice, Darth Vader.
When going through the Skywalker Saga, it's easy to see this rule illustrated. In The Phantom Menace, audiences were introduced to Sidious and Maul, but the latter is killed in the movie so his master takes a new apprentice in Count Dooku. A master and an apprentice." This refers to the Sith, who in contrast to the myriad Jedi throughout the galaxy, appears in scarce numbers.
The original phrasing of the directive, as Yoda first put it, is that "Always two there are no more, no less.
For years, fans took the rule at face value, but based on new details from The Rise of Skywalker novelization, there could be more to the rule than once thought. The "Rule of Two" is an old directive of the Sith Order that was first mentioned by Yoda in Star Wars: The Phantom Menace.