1,000 years before the Clone Wars, Sith Lords were common. but since then, that number has dropped down to two. by order of Darth Bane, who believed having more then two Sith Lords would just cause hopless fighting in the their own ranks. He turned out be right. And when the other Sith Lords died off, he brought his order into existence. One with only two, a Master, and an Apprentice.