Hi,
Easy Profile does not do this natively, but of course this is possible with a bit of code skill

Follow these steps:
- Download, Install and Enable Skeleton Plugin (you will find it at http://docs.easy-profile.com/index.php/article/triggers)
- Open file /plugins/jsn/skeleton/skeleton.php
- remove samples functions: triggerFieldAvatarUpdate, triggerProfileUpdate
- add function like this:

public function triggerProfileUpdate($user, &$data, $changed, $isNew)

{

  if($isNew) // Only when a user register an account

  {

     /* HERE THE CODE TO  */

     /* GENERATE THE NUMBER */

     $data['here_field_alias_where_store_the_value'] = $generated_number;

  }

}

this function will store in the field with alias "here_field_alias_where_store_the_value" the generated number.
Your number need to be incremented for each users, so you need to retrieve the last number stored, you can do this by using a sql query like

SELECT MAX(here_field_alias_where_store_the_value) as last FROM #__jsn_users