"This page or so that Xiao Wang saw" should be a grammatical error of the editor!

What it wants to express should be this: turn to the "left and right page numbers" of the book at will, that is, turn to the "left and right page numbers".

therefore

"The sum of one page or two pages that Xiao Wang saw"

It should be changed to "the sum of the left and right page numbers that Xiao Wang has seen"