Simple little addon to add a Stack Count line to the tooltip. No fancy configuration, so if you want to change the colors, you'll have to edit the numbers at the end of line:
Code:
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.