Five hundred plus is a recommended number. You may need to start with fewer and build. I began with only 50 books. Books can be checked out of the school library. Budget money for books rather than paper since it will no longer be necessary to do all those worksheets.
Some teachers check out books from the public library, others shop at garage sales.
The manner in which the books are displayed also impacts reading during this block. Students must be able to see the covers of the books if we want them to read. Remember, during self-selected reading the teacher is selling the concept of reading books. Conferencing Conferencing is the accountability factor in self-selected reading. The students will be more attentive to their books if they know they will be discussing the books with the teacher.