You can set up your listing with a multiple stock quantity if you have more than one of a particular product. When the item sells, your listing will automatically relist and the stock quantity will decrease. The stock quantity can be entered during the second step of creating your listing.
Please note, items with a stock quantity greater than one will only automatically relist if they sell. If the listing expires without selling, you will need to relist by locating the item in your Unsold list and selecting the Relist this item button.