Your site includes the option of Inventory Management. For every product or product option within your site, you can specify a number of available products. You can also specify a threshold of inventory at which point the site will automatically send you an email notification stating that the threshold has been reached. At that point, you can either log in and increase the available inventory or allow the site to continue to dwindle down to zero.
If the inventory of a product reaches zero, it will still appear within the site but it will no longer be available for sale. Similarly, if someone attempts to purchase a quantity of an item that exceeds available inventory, they'll see a message stating that insufficient inventory is available to meet their order.
You can choose to use inventory management for some products and not others.