To insert a page number field into a form (or report) you need to be in form design mode (or report view):
Select Page Number from the Insert menu.
The mouse pointer turns to a cross. Place this where you want the field to appear and click once on your mouse.
The page number field will be inserted at the chosen place.
In forms, click on the Browse button to switch to browse mode. As you use the Record buttons to scroll through the forms, each form will display its own page number. If you alter the order of the records by applying a sort order, the new order of forms will still be numbered consecutively from 1 upwards.
In reports, click on Print Preview. Each page will be numbered in order. Note that with a report it makes sense to put the page number in the Page Header or Page Footer.