Impact Items can be unlimited, or have a set quantity. Most times, having a set quantity can be useful if you have an online store (like you're selling 25 t-shirts for $20 apiece), or you're running a 'fund-a-need' style campaign.

To adjust quantity, head to the Impact Campaign you'd like to manage, and click "Impact Items". Now, click on the item you'd like to edit.

To make the quantity unlimited, leave Quantity blank. To adjust the quantity to a specific number, type that number in the Quantity field. In this case, I'll change it to 10 remaining.

Now, the new quantity remaining will update for your donors.