Inserting the page number field into a Writer document is easy.
VIDEO TUTORIAL: how to insert page numbers into Writer document
First you must understand that you insert page numbers into the header or footer of your document. This means you have to apply a header and footer to your pages.
The best way to do this is via the Style & Formatting window (F11). From the S&F window, click on the Page Styles icon, right-click on the page style you want to add a header/footer to and select Modify. A page style window will pop up, and fron the header or footer tab, you can activate the header for every page that uses that page style.
This may sound complicated, but trust me it's not.
You must also understand that you don't just type in a number into the header or footer--you have to insert the page number field. You do this by first placing your cursor inside your header or footer, then insert the page number field (Insert --> Fields --> Page Number). You'll see how easy this is to do once you actually do it.