"Always two there are, no more, no less. A master and an apprentice." ~Yoda, Star Wars Episode I: The Phantom Menace
I haven't read any of the Legends stuff involving Darth Bane, so I'm going to make up my own head canon in which he started this "Rule of Two" rumor to throw off the Jedi. The REAL rule that the Sith actually follow goes more like this:
"Always at least two there are. A master and an apprentice. And a spare apprentice or two in the wings in case the first one doesn't work out. And the apprentices' apprentices. And probably a bunch of Force sensitive younglings kept hidden from the Jedi just in case it all goes pear-shaped."
Not as catchy, I know. My alternate head canon is that Yoda is a lying liar who lies. From a certain point of view, that is. ;)