My dude has now witnessed a good number of uses of the scrolls to summon foul beasts. Though we'll wait to see if more spawn, the horde in Melhed was rather small for the amount of scrolls used... All of the spawns always seem to be about the same number of baddies, regardless of how many scrolls are used.
I suspect, however, that using a ton may either enlarge the area of effect or have a lingering effect, perhaps in addition to bigger spawns if bigger they ought to be. There may be a delay that must be considered as well...