I'm sure it varies a bit (both in length and in number) but, on average, how many poems does a chapbook contain?
This guide to creating a chapbook (PDF) suggests a length of 20 to 30 pages with no more than one poem per page, so roughly 25 shorter poems. If a poem takes more than one page, it would be inappropriate to start the next poem on the same page the previous poem ended, so assuming 12-point font, 8-inch high page, and roughly 1 inch top and bottom margins, one might fit 36 lines per page.
I personally feel that for very short poems, the restriction of no more than one poem per page is stingy. While fitting two sonnets with titles onto a single page would be cramped, two title lines and 16 body lines (including spacing lines between stanzas) might reasonably fit on a page even with a font size that is comfortable for relaxed reading. However, one may be unlikely to have two such very short poems which are close enough in theme to work on a single page.
Most presses that publish chapbooks give a range of pages they require. The smallest size for a chapbook I have ever seen was ten pages. That particular press called for a maximum of fifteen pages which is unusual; some refer to a chapbook that short as a “mini chapbook,” and some small presses specialize in them. They are likely to be more like pamphlets than books at that size range.
The average size is from 20-32 pages, but I have seen a press call for submissions of chapbooks asking for them to be from 30-42 pages. If you are self-publishing, 42 pp is also considered full-sized book length by some presses. General guidelines that are not press-specific are only useful if you are self-publishing, but what I’ve given here are the perimeters I’ve seen in the industry. As to how many poems to fit on the page, the standard practice is one poem per page, allowing an exception for two or more haikus or very short poems per page. However, if a single poem runs even one line over a page, no poems are usually placed on that page which contains the extra line. In other words, each poem must begin on a new page.
Chapbooks can also contain prose or mixed genres. The entire chapbook can be one essay or creative nonfiction piece, one piece of fiction, or multiple pieces of any of these. In this case, it is still common practice to begin each piece of whatever genre on a new page, regardless to however much empty space is left from the preceding piece.
Length of chapbooks is measured in pages, not pieces, be they poems, flash, or anything else. They can contain the work of one or multiple authors. They may include visual artwork, or not.