WARNING: SPOILERS FOR THE DARTH BANE TRILOGY.
There's only two sith at a time. I can think of a couple instances where Bane and Zannah both almost got themselves killed and thus ended the Sith Order. The trip to Tython, what if the hyperspace lanes had collapsed on them or the Jedi did succeed in killing them there? They could have both died in the Stone Prison as well. Or what if somewhere along the line the Master and Apprentice kill each other during their battle for succession? What if there is just some random accident that kills them both. Two people doesn't leave a lot room for error or bad luck.
Yes, this is the problem, the rule of two focuses the power of the darkside, but it also vastly increases the danger. If it was a small group, then it would work better, especially if they where spread out in twos or threes.
Mind you, the sith seem to have some sort of safety mechanism in place, after Darth Vader and the Emperor die, there should be no sith. Even when the Emperor returns, Luke kills all his apprentices until he becomes the Emperor's apprentice, before Leia saves him and they get rid of the Emperor again. Yet, years later the Sith return, and corrupt Jacen Solo, so they must have some sort of back-up system.