Edited to add: A quick count of the monsters in the Bestiary comes out to 143 monsters (I might be off by one or two). Koprus plus gray oozes plus nine types of demon is 11/143, which is about 8%. So technically speaking, yes, 5-10% of all monsters. But I don't think it counts when most of the monsters on that list are all variants of a single monster type. EA for pyromancers is basically "Your fire spells deal full damage to demons."
Look up Acid Resist, Cold Resist, etc.
I wonder if you will find that the percentage is even lower.