Page Numbers can be inserted in the margins either below or above.

Preferably on the outside of the page, that is to say: left on a left page and right on a right page. When browsing the book the page number is then easily visible on the outside edge.

Frequently we see the same mistake. Namely that on the first page of a chapter, which states the chapter title, page number, for aesthetic reasons is omitted. This page number is listed in the contents though, which of course is surprising. The reader can apparently find out himself what that page this is, by forward and backward scrolling.
In a commercially published book you would never find this.