Click Insert > Page Number, click a location (such as Top of Page), and then pick a style. Word automatically numbers every page.
When you’re done, click Close Header and Footer, or double-click anywhere outside the header and footer area.
Tip: Word automatically numbers every page, but you can change that if you want to. For example, if you don't want the page number to appear on the first page of your document, double-click or double-tap near the top or bottom of a page to open the Header & Footer Tools Design tab, and select Different First Page. For more options, choose Insert > Page Number > Format Page Numbers.