Summoning?

Hi, I'm just getting to grips with this game. I think I've got most of it down, but I'm still confused about summoning monsters.

Am I right in thinking that each summon spell can only be cast once at a time? I'd imagined that it might be possible to specialise in summoning to the extent that you could summon significant forces if you invested enough mana. But if you can only cast each one once then practically you're only likely to ever have a couple of summons - even as Procipinee! Have I missed something? Thanks!