Why doesn't my messages icon on my iPhone 5 display the number of messages?
You've turned it off in Notification Center.


Open Settings, then scroll to Notification Center and tap it.
Scroll down to the "Include" section; "Messages" should be there. If not you've turned off notifications for it altogether; scroll on down to "Do Not Include" and you should see it there. In either case, tap it.
Tap the "Badge App Icon" switch so it's on.


Your messages count should be back.