Is that supposed to be the # of the user that you seen when you click on a user's name and see their profile?
No, you need to specify the 'ID Number' of the user. That's a field you can fill by editing the user profile, with whatever information you want.
If you want to user usernames, just copy the username there. You can do this in a single go by executing the following database query:
UPDATE mdl_user SET idnumber=username;
Beware this will set the 'ID Number' for every single user in your Moodle site (including the guest and admin users).