I'm basing everything I know about the apparitions and wraiths off of the Kara no Kyoukai movies, and they usually appeared in groups of a dozen or more. They're pretty harmless individually, but a bunch of fairy apparitions working together could destroy a church pew in the sixth movie. Can't remember if the wraiths ever did anything too threatening besides being creepy.

EDIT: Good shit, Phase. Don't you think it's a little unwise to make our enemies stronger though? xD