Okay, so here's my thought on Sith power structure.

Regent(title should be Prince, rather than anything else because) there would be multiple, but very few. I'm thinking ten. There is one present on the expedition. The title "Prince" replaces Darth. 
Lords The top rank, other than regent, obviously. Carry a "Darth" title. 
Apprentice. Chosen from among Acolytes by a Lord. A Lord almost always has only one apprentice, but there has been a history having having multiple in secret, usually for the purpose of tricky plots. Carry a "Darth" title. They stay apprentice until either they die, or their master does. If their master dies, they replace them. If they die, they are replaced. 
Acolyte: The Sith will try to turn almost any being with Force potential, and bring them into the fold as Acolytes. Acolytes have no title, and are subservient even to lowly apprentices. 

Thoughts?