How long have you got? In some cases the additional apprentices are known to the Sith Master e.g Sidious was training Maul while Plaguis was still alive (though technically as a Sith assasin) and in The Force Awakens Darth Vader was training Starkiller as an Apprentice, telling him the plan was to overthrow Sidious.
How many are actually cannon now though?
As Disney have pretty much killed off all the Extended Universe that took place between Revenge of the Sith and a New Hope, along with anything that happened after Return of the Jedi I'm wondering what direction The Force Awakens will take.
I think we've gone off on a tangent here
Big tangent, but when a sith stops being taught is he not still a sith? Maul never died, he went in hiding. Douku trained ventress, then when she pee'd him off switched to oppress savage(although that's another tale).
So technically at one point there was Siddous, douku, maul, ventress and savage who could all claim to be sith lords(they knew the force to the extent of a Jedi master, but certainly weren't jedi).
No shock that the dark side breaks the rules........... And we are quite sad.....