Clone Cadets - S3-E1
Continuity mistake: During the first training test, when Fives and Cutup have an exchange where the former tells the latter to flank right and gets a bad pun in response, Cutup is wearing Echo's practice armour (with a number 3 on it) instead of his own, which is marked with a 4. (00:10:38)
Answer: Dooku never accepted Ventress as his apprentice, as far as I know; he just kept her as his devoted personal assassin. In Phantom Menace, it is stated that there can be only two Sith at a time, to prevent a power struggle in the lower ranks. To accept Ventress as an apprentice, Dooku would have violated that rule.