well theres so many parts in the later lore since tpm about the rule of 2 , first of the rule of two is explained in hte bane series ofc and explains it quite well, its mentioned in the tpm and through the prequals.
reading some of new jedi order and stand alone books there are vague references to it there, one book explaines how vergere was tested by sidious to become a sith apprentice but failed and she ran away to escape the assassins and then met up with the vong, so the lore about it is all over the eu.
im pretty certain in the bane books it says there can be minions of the sith but only 2 dark lords, so im guessing that means just 2 full lords of the sith but they can train and use as many acolytes or dark side users limited in power to help them out numbers wise to carry out the donkey work.
basically with the cloning technology the emporer had available and the essense transfer skill or whatever its called theres no reason to have the rule anyway, when body is oldish simply transfer over ot new cloned body, its a bit of a mess but i look that the saber wielders popping up in newer lore and just servents rather than sith lords