The most important thing in my opinion is nerfing the bonuses of the supports since if they are you don’t have to worry too much about how many supports characters can have.
I recommend the support’s boost be +1 for Mt and +2 for hit, avo, (crit avo,) and crit, maximum if the max amount of supports is 5. If the max amount of supports is 10 or 15 I recommend the supports boosts be +1 for Mt/Dmg and +1 or +1.5 (rounded down) for hit, avo, (crit avo,) and crit, maximum.
Basically the boost can’t be too high or too low for good balance.
I don’t think the amount of supports matter that much as long as the support bonuses aren’t too high.