As the designer you can set a limit to how many downloads are available to the customer for each design. To set the download limits for a design, go to the Info tab, then Customer Settings. Select the number of downloads you would like to allow for the design from the Max Downloads drop-down box:
You can also set the Max Downloads amount for all new designs by default in your account Settings. To set this number, go to Settings > Listing and Design Settings > Default Design Settings. Select the desired default download amount in the Max Downloads area.
If a customer runs out of downloads for their order and you want to add more for them, you can add or reset the downloads for any design in an existing order by going to the Orders page and selecting the order you want to adjust. Find the design(s) you want to add downloads to in the Item in Order area. Under the Downloads Left column, you will see how many downloads are available for that design. To edit that amount, click on the "pencil" icon next to the shown amount in that column.