To create a repeating data 'field' the quickest way possible, add a Content Control linked to a built-in property.

In your sample document, delete the formfields you added. Then go to Insert > Quick Parts > Document Property > Category. This will put a linked CC in your document. If you are showing your Developer Tab, you can change its title to 'Form Number'. Then copy and paste that same CC into the other locations. Try changing the text inside one of those CCs and when you move your cursor out of the CC, you will see the others instantly change.

I picked Category for this example, it doesn't matter which one you choose. If you have a limited number of fields that need to behave this way, you can repurpose any of these built-in fields the same way.

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Andrew Lockton
Chrysalis Design, Melbourne Australia