The level-dependent mob limit was introduced around three years ago when the max cap was adjusted from 8,000 to 3,300, and we’ve recently increased it to 4,000. Accounts created before that change retained their recruited mobs, but like all players, the number of mobs equipped in a fight is still restricted by level. You can check your mob count anytime under the Armory tab.
For more clarity, I’d recommend reviewing the game FAQ and using the "?" hint button—it’s a handy resource to understand the mechanics better! Let us know if you need further help.
Walle Berg