A rule to protect their own self interests. Too many Sith, too many power plays and backstabbing. So just two. A master and an apprentice. When the master no longer can teach, the master is killed and the apprentice becomes the new master. If the apprentice is not ready, they will either be punished by the master, or if the master thinks it is time to get someone else who might better his own cause, kill the apprentice. Or go the Palpatine way and engineer a situation to get your apprentice killed.
Its still a lot of bollocks - crazy power-mad people think they can control situations - it would never hold.
It will not last on screen anyway now they are going to be corning these out every year - at some point we will see an army of sith coming over the hill.
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