<div class="paginate_box">
  <span class="disabled prev_page">Back</span>
  <span class="current">1</span>
  <a rel="next" href="page2">2</a>
  <a rel="next" href="page3">3</a>
  <a class="next_page" rel="next" href="page2">Next</a>
</div>

doc = Nokogiri::HTML(html)
doc.xpath('//div[@class="paginate_box"]/a[not(@class="next_page")]').last.text
#=> "3"