The easiest way to think about pages is to think about a PDF document with multiple pages. Each page in that PDF document would be equivalent to one page in the finished, bound book. A sheet is what the pages are printed on. For saddle stitching, a sheet has 4 pages on it, and therefore the page count must be divisible by 4. For Spiral, Wire-O, Perfect Binding, and Corner Stapling, a sheet has 2 pages.