Most of the self-produced chapbooks I've seen from local poets run between 20 and 60 pages.
It depends how you want to bind it. Stapling is cheap and easy, but works better with slender books. Be sure to have them cut the pages so that they are even widths once stapled.
It also depends whether you are selling this, for how much money, and where.
I would go to an artsy bookstore or coffee shop that supports independent poets and browse for yourself, if you can.