Hi @Exort. The limit on summons is part of the boon itself, and was added due to the power of what we call “action economy”, where many actions, even lower powered ones, can start to take over an encounter with increased options, spread of damage and chance of explosions.
You can see the rule in the Effect section of the Summon Creature rules, but I’ll paste it below anyway:
Your minion cannot act on the turn that it is summoned. On each of your following turns, your minion acts on your initiative count according to your direction, receiving the usual assortment of actions. You are limited to a maximum number of summoned creatures equal to your invoking attribute score. Feats and other abilities cannot increase this limit.