The quantity should display by default, if the quantity is not displaying, this is due to the products not being properly configured to have an inventory. 

All store products need  to have an inventory level: Fundraising > Ecommerce > select Edit on the product in question > Step 14. Manage Inventory > select " Yes, track inventory for this product. " > add quantity on hand (you can optionally set up threshold quantities).

If there are configuriation options for the product, each configuration option will have its own inventory numbers (such as a Large T-Shirt).