Spells summoning multiple creatures of the same type only create one creature
Although impossible to create in game, it is possible to create spell in the CS that contains multiple magical effects of the same type. Jemolk was kind enough to test this in OpenMW in the case of summons. To quote him:
This should be simple enough to check; a modded spell could do it. In the CS you can add the same effect more than > once. In fact, in Uvirith's Legacy, there's a belt that has a constant effect to summon around 6 Skeletons. I'll go > in game right now, console-add it, equip and check what happens! EDIT: Okay, back! The belt only summons 1 Skeleton, despite having 7 copies of the "Summon Skeleton" effect.
This is not how the mentioned item is intended to work. We also need to check other spell effects in the game.