I am thinking of making a book of poetry and was wondering how many poems do I need?
Books under 50 pages, particularly if they are saddle stapled or bound in some similar style, are generally referred to as chapbooks. Based on what I've seen of publishers requirements for submissions, these are generally in the 16 to 44 page range (my estimate, not official). Many poets, especially new and emerging, publish chapbooks first. Longer ("full length") poetry books are usually perfect bound and can range from 60 pages upwards. Notice that I am using pages rather than number of poems. It is one poem per page, but if your work is longer, a poem may take multiple pages.
I would recommend buying a few books of the type you like to get some ideas of how large a compilation you want to make and, if you are publishing it yourself, what type of binding, paper stock, etc. you like for your book.
If you self pub, places such as Amazon will print a book as low as 24 pages, so if your poetry is an epic poem it can be almost 1 book itself, say like Beowolf. Font size and how you typset it also play a role. If this is for commercial or your own use? I would at least think 3 poems filling 24 pages is small, but doable. They are called booklets.