If you use a stack of heads (Ex: Holding a stack of Zombie Heads) and right click an armor stand, the Armor stand will take all 64.

(Minecraft Snapshot 1.8) I thought that if you Right clicked an Armor Stand while holding 64 Wither Skull Heads, it would simply remove 1 Wither Skull Head leaving you with 63. Instead, The Armor Stand takes ALL of the Wither Skull Heads. Switching the head with something else gives you back your Wither Skull Heads.