The Rule of Two was only a thousand years old. The Jedi has mostly thought they had killed the Sith centuries ago, but managed to miss one of them each time it would seem.
Prior to Darth Bane, a thousand years ago, there were lots of Sith Lords, and they managed to backstab each other too much so that the Jedi were able to defeat them.
Also we know the Sith cheat their own rules by having Dark Siders that work for them, but are not trained as Sith. There are only two Sith Lords, but they tend to take on some underlings while they are at it. Also it seems to be common for the Sith Apprentice to have a "secret" apprentice of there own that they are potentially grooming to be the next Sith Lord, after they do away with their Master.
For a short time I think the Jedi thought that the two Sith Lords were Dooku and Ventress. But I could be wrong about that.
Prior to Darth Bane, a thousand years ago, there were lots of Sith Lords, and they managed to backstab each other too much so that the Jedi were able to defeat them.
Also we know the Sith cheat their own rules by having Dark Siders that work for them, but are not trained as Sith. There are only two Sith Lords, but they tend to take on some underlings while they are at it. Also it seems to be common for the Sith Apprentice to have a "secret" apprentice of there own that they are potentially grooming to be the next Sith Lord, after they do away with their Master.
For a short time I think the Jedi thought that the two Sith Lords were Dooku and Ventress. But I could be wrong about that.