You can now have up to 5 levels of sub pages.

 To nest a page within a page, simply drag and drop it into your desired location from the Pages menu on the right-hand side of your screen. 

More info here https://gsuiteupdates.googleblog.com/2017/10/expanded-page-hierarchy-in-new-google-sites.html

Example: Look at page 4 here: https://sites.google.com/view/multipagetest