I am willing to negotiate custody of the necromanced mermaids, but it will come at a cost. Secondly, the mermaids are under the mirror's thrall, so you are fighting that influence, meaning they'll require more exertion to be pulled under Wei Wuxian's control. Thirdly, mermaids are not human, so I want an imperfect control of them, based purely on the fact that... they probably can't communicate or resonate as well with Wei Wuxian on that basis.

So I can give you:
1. three mermaids, for four-five hours, 90% control
2. five mermaids for three hours, 80% control, Wei Wuxian is absolutely exhausted for a day or two.
3. eight-ten mermaids for one-two hours, 75% control, Wei Wuxian is dramatically out of it for two-three days, fainting, salts, bring a chair, this man is pregnant, the entire spiel, possibly with backup dancers. No magic for him for days, no bouncing around, nothing. He is useless during the recovery period after this.

Pick an option, if you'd like to go ahead, but know that I can't promise a peaceful follow-up to the event.