if you want it to be shown on button, you have to edit each food button caption in menu. but you can put another button that will show stock of selected product or you can create actions and rules that will stop selling out of stock items.
You can visit following topic Stop selling out of stock items
However, you are not able to fully control to stop out of stock item sales even if you put those rules, because there other ways to mistakenly place and order of out of stock items