Once you've added a product to your store, you can adjust the inventory to display the number of stock items you have.
You'll have the options to:
Add more stock to your inventory
Reduce the number of stock you have
Log a return to the inventory
Log a cancelation back to the inventory
To adjust your inventory settings, head to Store ➔ click on a Store Item ➔ click on "Options" next to the variant you want to adjust the stock for ➔ Inventory Details ➔ Adjust Inventory