how do I create a mailing label from a guest list created on Apple Numbers?

If do not want to put your data on a third-party website, you can also stay all Apple.

Import your data into the Contacts app and print labels from there.

More information here:

Import contacts into Contacts on Mac - Apple Support

Print mailing labels, envelopes, and contact lists in Contacts on Mac - Apple Support

SG

The method that works best for me is to use Numbers File menu > Export to export the spreadsheet to Excel format and then use the free and very good Avery Design & Print online to merge the Excel spreadsheet. See: https://www.avery.com/software/design-and-print/