When a product's inventory is tracked in Shopify, the inventory quantity that's available at a location is updated automatically when you receive a transfer that includes the product.
From your Shopify admin, go to Products.
Click the name of the product that you want to track.
If the product has variants, then click the variant that you want to track.
In the Inventory section, check Track quantity.
If you want to allow customers to purchase the item when it's out of stock, then check to Continue selling when out of stock.
In the Quantity section, enter the quantity information for each location.
Click Save.
The Continue selling when out of stock checkbox doesn't apply to Shopify POS. Staff can continue selling products when available inventory reaches zero and below. POS will warn staff before they sell an item that’s not available.