A family of Microsoft word processing software products for creating web, email, and print documents.
If you are inserting the header and page number as building blocks, then each will replace the other. There are several alternatives. The first and simplest is just to double-click in the header area and type/insert whatever text you require. You can insert a PAGE field by pressing Alt+Shift+P.
If you want to insert a page number in a header building block that doesn't include one, place the insertion point where you want the page number and press Alt+Shift+P to insert a simple number or, if you want one of the fancy ones, use the Current Position setting for Insert Page Number and then choose from the gallery.