This has gone sort of off-topic lol, anyway 2 per caster would be nice IMO, or create some anti-summoning spells that deals a lot of damges to summons so even if you summon a lot, they die very quickly(warcraft3 have this kind of spell/items to destroy necromancer armies that summon a LOT of skeletons).
There are ways to balance things out instead of limiting them. Take the old westwood game Nox for example, summons are limited in slot, you can have at most 4 summons per Conjurer, but summons that take 1 slot are weak. So you can choose to have 2 weak summons+1 medium summon, or one big summon that take all 4 slots.
PS. I'm just giving out ideas. It's ok if it's hard-coded in A5, it's still a great game.