You usually take the page number for granted. It doesn’t even figure in a famous book on typography but a lot of thought goes into where the page number should be on the page. It was Jan Tschichold who created the simple page number design as we know it today. A lot of things should be taken into account while creating a page number; for instance, eye movement is taken into consideration. Then other factors including where the page number is first placed, the odd and even placement of numbers on the verso and recto, the bookbinding requirements, etc.