At the end of page 15, insert a section break. Go to the page numbering in the first section, i.e. the footer (or header). Hit the keyboard shortcut Alt-F9 to show the codes. It should read something like
Page { Page } of { NUMPAGES }
Change that to
Page { Page } of { SECTIONPAGES }
and the second number will display the number of pages in the current section after you refresh the fields with F9 and toggle back field codes to normal display with Alt-F9.