Good day, Beylee,
You can add item numbers on the Product/Service item you'll create. Then, on the Invoice window, click the Gear icon, and check the box for Item/Service code under Optional Columns.
However, if it's different for each customer, you'll want to make use of the DESCRIPTION box or Message on invoice box to add them.
Check these articles for future references:
Let me know if there's anything else I can help you with.
