In your Maker Portal, you have the ability to manage inventory using "Track inventory" or "Do not track" for each product:
All products default to "Track inventory," so be sure to change this if necessary. (As a Maker who crafts products per order, you may not need to track inventory.)
When tracking inventory, simply add to the "In Stock" field under Pricing/Inventory within the product listing or by inputting an amount under "Quantity" when viewing All Products (see below).
Remember, there are other ways to control how buyers order by using your Minimum Order Amount in your "General" account settings, or by using Minimum or Maximum Order Quantity amount at the product level.