Technically, Ventress is Count Dooku's apprentice.  However, Dooku was Palpatine's apprentice at the time...

A lot of this is covered in [The Rule of Two][1] entry on Wookieepedia.  TL;DR - Palpatine considered it more like what you'd call "guidelines" than an actual rule, really.


  [1]: https://starwars.fandom.com/wiki/Rule_of_two