If during adding a new product you have selected the option to include the new product in parts inventory...
...and hit "Save", you'll have an option to edit the product's quantity in your stock. Providing you have ticked Trackable Inventory and have entered MIn Stock Level and Max Stock Level
Just click "Edit Stock Quantity" field...
...put the correct quantity and hit "Update"
Optionally, when adding stock quantity, you can create a purchase order for the items that are being added.
Just go to "Purchase Stock" section on the same panel, add a contact name and the desired quantity (1)
Value, account type, tax rate and the total amount will be pulled through automatically
When you're sure, that everything's correct, click "Commit"
After updating the stock quantity, it will be shown next to the product on the "Inventory" list:
From now on every time when committing an invoice with the stock product, the invoiced quantity will be subtracted from the total.
Also whenever you add a purchase, the stock count will increase
To add an already existing product to the inventory, simply select it from the products' list:
On the product details page, tick into the "Trackable Inventory" box (1) and save it (2)
Then follow exactly as with adding new products to the stock including updating quantity and optional creation of a purchase order