Do not attempt to do this for the normal blank document that opens when you
start Word: that changes the global template which will have undesirable
consequences in unexpected places.
Instead, you need to create a template that includes the page numbers (and
any other changes that you need such as paper sizes, fonts, tec.) and save
to your template folder. You can then add a shortcut to your desktop so that
Word opens to a new blank from your template or you can use File, New and
select a blank document from your template.