Steve Tippins Ph.D. tells graduate students to stop obsessing over page count. A dissertation should be as long as it needs to fully tell your research story, show you understand the field, and prove the work is complete.
Chapters
Please stop chasing page counts. I've seen too many people say, "Ah, I got my 40 pages, I'm done," but they haven't fully explained things. Or, "I got 40 pages, but really they only needed 35 to say what they needed." So yes, pages have to be there, but ultimately you have to be telling your story, and whatever is needed to tell your story is the appropriate number of pages. And that appropriate number of pages is there so that you can A) show that you've done what you need to do, B) show you understand your field, field, understand the literature, how your work ties into all the work that's done before you. Because work doesn't happen in a vacuum. You are— become part of generations of research. So you want to be able to let the reader know that you understand what you've done and how you fit into the overall scheme of research in your field. I'd love to help you finish your dissertation so that you can go out and change the world.