One Sith had the cunning to survive. Darth Bane restructured the cult, so that there could only be two -- no more, no less -- a master, and an apprentice. Bane adopted cunning, subterfuge, and stealth as the fundamental tenets of the Sith order. Bane took an apprentice. When that apprentice succeeded him, that new Sith Lord would take an apprentice.
this is taken directly from the starwars databank, and is not EU it is from the movies section