WARNING: SPOILERS FOR THE DARTH BANE TRILOGY.
I mean don't get me wrong, Darth Bane fracking awesome and his plan did work, but it just seems so risky. 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. Anyone else agree?
It's less risky, or at least no more risky than having an army of followers which could potentially turn on you at any moment. Maybe it wouldn't work as a universal rule and I doubt Bane meant it to be, but sometimes circumstances call for a more subtle approach than just trying to overpower the Republic.
Besides, the Sith are pretty darn exclusive. Ideally the point is that a handpicked apprentice to the Dark Lord of the Sith will have shown himself capable of handling unforeseen "accidents".