Add stack count to the tooltip with this free add-on for WoW
Version:StackCount is a simple little add-on to add a stack count line to the tooltip. It provides no fancy configuration, so if you want to change the colors, you'll have to edit the numbers at the end of line:
Operating System: Mac OS X
GameTooltip:AddDoubleLine("Stacks in lots of: " ,itemStackCount, 0, 1, 1, 1, 1, 1, 0)
The tooltip will only show if the item selected is stackable.