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.