How to automatically assign a (sequential) number to a user-created node
The Serial module allows you to do exactly that. You can simply have an integer number that is incremented per content type and if you wanted a prefix you could do that in the display formatter.


  Unlike Drupal's built-in auto-increment node ID, which is global and
  shared by nodes belonging to all content types, serial fields are
  managed per content type (D6) / entity (D7). For example, the serial
  field of an Invoice instance will generate a unique sequential number
  (starting at 1, then 2, etc.) exclusively for Invoice instances.