User blog comment:Imhungry4444/Fairy Tail Wikia Podcast 10/19/13/@comment-24039250-20131022192447

One of the things I noticed was that it's believed that there is 1 demon per book since Carla said it (though when does she get things absolutely correct, Ex. GMG (her "predictions" were useless)) but I kinda feel that was just a belief, since there would have to be 10K+ books since it was rumored in the beginning that Zeref created over 10k demons in the war 400 years ago.