Keep this in your students reading folder to keep track of how many books they finished. Many of my students struggle to finish a chapter book simply because they just want to get a new one out. This encourages them to finish a book to see how many they can finish in a month.
Have students set a goal at the beginning of each month to guess how many books they think they will be able to finish. Many of them will guess very high numbers to start and eventually will realize their guesses may not have been realistic. Every time a student would finish a book, they got to color in a box. At the end of the month, see how many they actually finished.