Page numbers in Atticus cannot be customized to appear only on the left or right side. Page numbers automatically placed in either the header or footer based on your theme's Headers and Footers settings.
If you choose to place page numbers in the header, they will appear on the outer corner of the page (the left corner for left facing pages and the right corner for right facing pages). However, they will not be displayed on the first page of any chapter or special page. This is because 'chapter heading' formatting takes precedence on those pages.