We're up to 7 players, 5 of which use some form of spellcasting. Avoiding the BBEG in favor of multiple enemies and/or goals gives them different stuff to do. House ruling some cooperative magic helps, too. Two wizards casting weapon 4 fire attacks is fun, but two wizards combining for a weapon 8 fire attack is even more fun. (it also might be a little over-powered, as I just allow them to stack their evocation power ratings together after each paying the 1 mental stress. But I make one assist the discipline roll, and one control it, which means someone has to pull off a fairly hard check if they do it).
We have a full wizard, a crafter/thaumaturge, two different sponsored magic users, and a lesser thaumaturge, and no one complains about it at all. They just decide who handles what and go from there: For example, someone might focus on saving the babies from being sacrificed, another might focus on the enemies, another will protect the group, someone else might try to disrupt the enemies' ritual, and the last one might go out for pizza. Or veil the whole battle from normies.