Enable headers/footers on your template (depending on where you want the page numbers to go)
Drag the 'page number' block from the Header/Footer Features section and place it on the template
If your template only has a single page (i.e. you did not add more pages from the Page menu at the top), you don't have to do anything else and page numbers will be added to any CV using that template on export
If your template has multiple pages, you need to repeat the above steps for every page you create if you want page numbers to be on them.
Please note that the page numbers in the template builder are only an indication. Accurate page numbers are rendered on export.