There are only two Sith, one master and one apprentice. One to embody the power and the other to crave it. This decree came from Darth Bane.
In the case of Asaaj Ventress, she is a dark jedi/ Sith aspirant. Another term is that she is a dark side adept. Ventress is also Dooku's "secret" apprentice. She is Dooku's assassin, not a Sith. Dooku/Darth Tyranus hopes to use her to beat Palpatine/Sidious. Only then could Dooku make Ventress his true apprentice. However, she is no secret as Sidious realizes that Dooku is training her in a future attempt to kill him. Darth Sidious orders Dooku to kill her, but she survives.
I highly recommend Jedi Vs. Sith: The Essential guide to the Force.
