An item can have 4 different spells attached to it. That summon-spell can have 2 summons, so that's 8 total summons.
The one thing you need to keep in mind, is that only the last spell added to the item may have a cooldown (all the other cooldowns must be 0 sec).
So if the item has SummonA, SummonB, SummonC and SummonD.
Then SummonD can have a 20 sec cooldown, but A, B and C must have a 0 sec cooldown.
This will result in the item having a 20 sec cooldown, yet spawning all the 8 monster types.
That's a great idea, actually .It's also possible to put a spellbook in the item. That way the amount of spells is quite about unlimited.