According to dlsite, the actual contents should be 23 pages (本編23ページ, "main story 23 pages"), not counting covers, printing credits, or blank pages. There are currently only 21 pages (3-23), not the mention the usual atogaki/あとがき/afterword is also missing.
Compare with the previous work in the series, Yuukarin ga Seme Sasete Kureru Hon 3, which dlsite says has 22 pages in its main story (本編22ページ), which lines up with pages 2-23, again not counting the covers, afterword, or credits.