WP Box

You can show how many comments add by registered users on your blog. Try it.

1) Copy this code

<?php
global $wpdb;
$where = 'WHERE comment_approved = 1 AND user_id <> 0';
$comment_counts = (array) $wpdb->get_results("
  SELECT user_id, COUNT( * ) AS total
  FROM {$wpdb->comments}
  {$where}
  GROUP BY user_id
 ", object);
foreach ( $comment_counts as $count ) {
  $user = get_userdata($count->user_id);
  echo 'User ' . $user->display_name . ' comment count is ' . $count->total . '
';
}
?>

2) Paste it where you would like the count to be displayed on you wordpress blog.

you can re-arrange the code as desired.

Done.