"When your power eclipses mine I will become expendable. This is the Rule of Two: one Master and one apprentice. When you are ready to claim the mantle of Dark Lord as your own, you must do so by eliminating me." ―Darth Bane, to Darth Zannah
As I've mentioned in a previous answer, I feel that many people take The Rule of Two far too literally. I believe that
It can be interpreted that Yoda isn't saying that there are only two Sith ever, but that there are always two Sith working together: a master and an apprentice. If one Sith shows up, there is "always" another one involved.
The Rule of Two seems to exist in C-Canon, being featured in a few books, but to my recollection is never actually defined or even mentioned in G-Canon.
MACE WINDU : There is no doubt. The mysterious warrior was a Sith.
YODA : Always two there are....no more...no less. A master and an apprentice.
MACE WINDU : But which one was destroyed, the master or the apprentice?
Aside from this single vague mention in Episode One, does the Rule of Two actually ever figure into any G or T Canon materials, and is it ever actually defined?