This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
That map is not counting threads, it's counting servlet instances. Your web container may create only one single instance that will handle all requests. It also does not count actual instances, only instances that are accessed. If a servlet instance is never accessed this code will never be called.