Assign random (and unique) user meta upon registration

You can use the hook user_register to modify user data upon registration. You can use the function add_user_meta to add custom data to a user. Example:

add_action('user_register', 'add_pin_number', 10, 1);

function add_pin_number($user_id)
{

   add_user_meta( $user_id, 'pin_number', $random_number , true );

}

You can look for php function out there to help you creating the random unique number. One idea is to create the number based on the user ID which is already unique.