During his appearances on Star Wars: The Clone Wars, Dooku instructed two Sith apprentices, Asajj Ventress and Savage Opress. Offscreen, the Jedi Quinlan Vos also pledged loyalty to the Count, as described in the canon novel Dark Disciple. All of these characters were powerful Force users and warriors, made all the more deadly after training with the Count. Ventress especially became so effective that Sidious ordered Dooku to eliminate her, concerned that her power could unbalance the sacred Sith Rule of Two. Yet Count Dooku almost gained a fourth apprentice, who would likely have become more powerful than any of his others.
Source：https://www.cbr.com/count-dooku-pong-krell-sith-apprentice/
