After the great Sith War when all the Sith banded together, Darth Bane destoyed them and made the rule of two -Two there should be, one to embody the power, the other to crave it.- The sith after Darth Bane followed this rule to the letter.
In the time between the founding of the Sith Order and the Seventh battle of Ruusan c.1003 BBY, there was an order of Sith, just like the Jedi. After Darth Bane betrayed them and killed them he created a new order adhering to the Rule of Two. When Banes apprentice Darth Zannah took her first apprentice, he didnt agree with the Rule of Two and so he abandoned the order and became the leader of a Sith cult. The Rule of Two was followed from then on by all Sith until at least the time of Darth Sidious. However, after the Sith-Imperial war, Darth Krayt rose to power with a new idea, the Rule of One. This meant there would again be an order like the Jedi, but there would be a single Sith Lord who all others would follow.