Commerce will generate a licence key upon a purchase whereas Single Use License Keys app can pre-generate a key "ready to use" either with or without a sale item.
A new product (or editing an existing one) via Commerce app and choose to use the Single Use License Keys licence method. The next step is to view the Single Use License Keys application itself and select 'Import Keys' and the product. You then add the keys (copy/paste recommended) and click to complete. Single Use License Keys will then adjust the stock levels to ensure that a client cannot purchase the product if there are no licence keys available for it.
The overview page in the Single Use License Keys application will let you see the total number of keys that have been already issued and any remaining (free) keys. You can also from this page choose a product and examine its key status too.