Mail Merge in Mac

Pages v11 has no built-in Mail Merge capability. It might be done programmatically, but otherwise, you probably should be using Word and Excel, or the free LibreOffice Suite (writer and calc) by exporting your Numbers content to Excel.

If you really want to stay with Pages and Numbers and you're willing to tinker a bit with scripting then you can get mail merge going as described in this thread.

SG