If you kill a group of wither skeletons with charged creepers, It only drops 1 wither skeleton skull. This is the same if you kill 1 wither skeleton, 2 wither skeletons, 3 wither skeletons, any group amount to my knowledge drops only 1 wither skull. Turns out, this is true for all skulls, and It does not match up with the bedrock version, where all of them will drop their skulls