I’m not sure what the “item number” is that you’re referring to, so to remove doubt, could you include a screenshot of what you mean? You can upload the screenshot to your media library and share the link with us, or you can use a service like https://droplr.com/ or https://www.getcloudapp.com/ and drop their link here.

If what you call the “item number” is also referred to as the SKU, then a variable product should have 2 places where that can be added. One is at the parent product level under Product Data ? Inventory:

… and the other is on each variable under Product Data ? Variations ??Select a Variation:

Also have a look at the Variable Product documentation for further details.