The extent of a book is a measure of the number of pages the book has. It includes every leaf in the book ~ that is the amount of paper, regardless of content matter, preliminaries and index. Since every leaf has two sides the extent will always be an even number.
Traditionally books were always printed on large sheets and folded down ~ and so the extent would always be divisible by sixteen. The term is not often used outside the publishing trade, and I caution against any form of page counting which does not clarify the exact meaning. I have read numerous reviews of online purchases where buyers complain that the number of pages was half that advertised. Extent and 'number of pages' surely belong in an area where true meaning are misunderstood ~ or worse ~ misinterpreted. I use extent loosely to mean the thickness or fatness of a book in contrast to the slimness ~ as in the phrase a slim book'.