ShopSite Pro level stores have inventory tracking where you can specify quantity on hand, Low Stock threshold:, and Out of Stock limit:.
If you have ShopSite 11 sp2 there is an option to display different text based on the quantity on hand as it relates to the Low stock and Out of Stock limits. So you could have it say Only 10 left when the quantity drops to the Low Stock level and Out of Stock when it hits the Out of stock limit. This is only possible on the product More Information pages, which is probably the place you want it anyway. To display this information you use the tag [-- MoreInfoInventoryActual --] in the moreinfo page section of the product template.
In older version there were also some tags to display inventory in real-time using server side includes, if you host allows this.
You can also do it at publish time by checking the quantity on hand field and if it is 0 then have your product template display your out of stock message instead of the add to cart buttons. This method can be used on any page where the product is displayed instead of just the moreinfo page like the tag mentioned above.