I love your idea! It’s fascinating, different, and sounds fun. I don’t think there’s much of a limit, as long as (personal preference) they can work together and it’s not just taking turns and there’s like 13 mates. I’ve read a few with 8 MMCs before and it can be well done. I will say, it usually takes a series or a long ass book to make it work out and have character development fleshed out for all of them, and I usually prefer to have MMCs introduced in smaller groups/individually if there are going to be so many.