Complex sentence: The manuscript measures, with hundreds of vellum pages collected into eighteen quires; depending on how some of its unusual fold-out multi-part pages are counted, approximately 240 pages in total.
Split into simpler sentences: The manuscript measures, with hundreds of vellum pages collected into eighteen quires. It is made up of, depending on how some of its unusual fold-out multi-part pages are counted, approximately 240 pages in total.