Board Thread:Questions and Answers/@comment-38413232-20190311233813/@comment-28802873-20190419091702

The reason Ben can't use more than 10 might be because whenever he got near to 11, something meddled. Like, even in the first episode, it's implied that Ben had 9 aliens only, until he unlocked Overflow, calling him a 'new guy'.

When Ben unlocked Gax, he unlocked him in an unusual way because of Vilgax getting near, he could've unlocked another alien normally.

When he unlocked Shock Rock, it was after that the Omnitrix was rebooted by Upgrade's doing.

When he unlocked Humungousaur, Rath and Slapback, it was after Phil's tool somehow triggered the watch to function again after another Omnitrix reboot.

Ben never unlocked new aliens normally in the reboot because whenever he got to 10, something happened afterwards. Maybe if he meets Azmuth, all of this can be cleared up, but for now, whenever he's stuck at 10, it's because something happened beforehand.