You can only have 5 summons at any given time, not five summon spells. If you cast Monster Summoning 2, for example, and summoned 3 creatures, and then cast Monster summoning 3, and summon 3 creatures, it will not summon one of them. I believe that the unsummoned creatures still count toward your limit, but I'm not sure. I have heard that Project Images get to summon more, but I've never tried it before.
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