You don't necessarily need multiple copies of each of the 15 books to have students participate in reading and voting.
In my library I only have one copy of each title. Since the majority of students who will be participating are in Kindergarten, 1st and 2nd grades, we are enjoying the titles as a whole classes.
Part of my reasoning for this is financial, but mostly I want all of my K-2 students to be able to participate in the voting in late February. Students only need to have read (or have read to them) 8 of the 15 titles to be able to vote.
I may buy a copy or two more of a few titles depending on the demand after we read them together. Even my Kindergartners know how to place holds - and when they like something they do!
Since for some of the books it will be a long time between the "read-together" and voting, I am trying to incorporate some ways for us to remember the books come voting time.
After we read the book together, I asked students what they liked about the story, or what they will remember and then recorded their answers.
Here is what they said: