The Product quantity can be set in the Product settings. Here's how you can do this:
1. Click Catalog and then Products.
2. Click on the product which you want to track.
3. Under Stock Control, click Manage.
4. Fill in the quantity or select the Unlimited option (this means the product is always for sale and the product quantity will never be decreased to 0).
5. If you want a notification for when your stock reaches low, fill in the number at which point you want to be alerted.
6. Click Save.
If you set product quantity, it will be automatically reduced after each purchase until it reaches zero. After that, the product gets Out of stock status and will not be available for sale.
Hiding Out of stock products
You can hide Out of stock products in your store using Hide out of stock products option. Here's how you can do this:
1. Go to Settings > General.
2. Click the Cart & Checkout link.
3. In the Catalog Settings section, tick the box next to Hide out of stock products.
4. Click Save.