Make sure you've already added products to your store. If you haven't yet, click here to Learn How to Add Products to Your Online Store
Step 1 - Select the Product
Click the "Store" tab on the left side menu, then hover your mouse over the [store] icon, then select [products].
Click on the product's  icon
Step 2: Set a Product Inventory Limit
Scroll over to the Price & Variants section and check the [Limit product inventory] checkbox.
Enter a value for the quantity
When a customer makes a purchase, the value of the quantity is automatically updated.
When the value reaches "0" the product page will display the product as "Out of Stock"
Simply update the quantity to increase the limit
Step 3 - Don't forget to save!