StackCount

GameTooltip:HookScript('OnTooltipSetItem', function(self)
    local _, link = self:GetItem()
    if link then
        local _, _, _, _, _, _, _, count = GetItemInfo(link)
        if count and count > 1 then
            self:AddDoubleLine('Stacks to:', count, 0, 1, 1, 1, 1, 1, 0)
        end
    end
end)