If Sith Apprentices always kill the master and take their position, why do Sith Lords keep taking apprentices and teaching them their ways?
The Darth Bane trilogy of books explains the concept of the rule of two very well, for starters there are only two Sith because that way the power base is focused.
There is not a huge array of Sith fighting each other for power, as is the nature of the dark side and so it is harder for the Jedi to confirm their existence, thus ensuring the continued survival of the true Sith.