*are not the only dark side users

Darth Bane, Darth Plagueis, Darth Sidious, and Darth Kryat where all working on a way to achieve immortality. Soul Transfer, I think it was called, where they essentially move their essence to another body.

Bane’s Rule of Two makes a good amount of sense. IMO.

Pretty much the only time the Jedi actually lost was due to the Rule of Two.

Regardless, Sith of the past have tended to keep less powerful dark side minions on hand. Ventress, for example.

Darth Tyranus (not Tyrannical) is Count Doku. Doku left the order and became a Sith after Maul’s “death”. This is supported by the erstwhile EU, and the new EU under Disney. Remember Ep I, and Ep II had quite a long time in between them.

Sith are the only dark side users. Dark Jedi, rouge force users, Night Sisters (cannon from the Clone Wars), and many other dark side force users exit. The Rule of Two only applies to the Sith. Which are something of a religous order, as opposed to the enbodiement of the dark side.

There was quite extensive mention of it in the EU. Hell, there was an entire trilogy that told the story of Darth Bane. The Sith lord who destroyed the Brotherhood of Darkness (lots of Sith) and instuited the Rule of Two.

“Two there should be; no more, no less. One to embody power, the other to crave it”