There are 10 used in the entire game, one is in the wareshouseambushes (resource cell, not playable), one inside Labirinthian, and one is locked up in blackreach. So 7 dotted around skyrim (not counting the pale lady). I've found four while playing - the weirdest coming out of a well in the middle of nowhere in the fall forest area with no prior warning. They're also tied to a levelgate so won't appear until lvl 15. So yeah, Skyrim could use some more wispmothers. I'm kind of tied up right now but if nobody else has picked it up in a few weeks I'll have a go at placing some more in interesting locations. Maybe even add a little quest and some backstory - the book The Wispmother found ingame offers some interesting hooks.