add serial id to content type
The Serial module will be helpful for you.


  This module provides an auto-increment (serial) field.
  
  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.