I tried to kill a large group of wither skeletons ( standing on the same block) with a charged creeper all at once to get lots of wither skulls, but they only dropped one. I hopped into a creative world and noticed that every time a charged creeper killed more than 1 mob it only drops 1 head. If 2 mobs are not on the same block it still only drops 1 head/skull. If different mobs are used it still only drops 1 head or skull.  (tested with zombie and wither skeleton). The head/skull that is dropped seems to be related to the coordinates of the killed mobs with relative to the charged creeper.