system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Why can there only be two Siths?

And why was Dooku trying to make Ventress into a Sith?
gpt
At the end of the Jedi-Sith Wars, Darth Bane was the only surviving Sith Lord. He knew that the infighting among the Sith was a major cause in their defeat. So in order to hide the fact that the Sith had survived, he came up with the Rule of Two. The Sith shall always only exist as two, a Master to embody the Dark Side of the Force, and an Apprentice to crave it. 

This rule allowed the Sith to grow again covertly. It resulted in Darth Sidious, who managed to become the Galactic Emperor and annihilate the Jedi Order.