How to calculate total number of comments made by a particular user

You can use get_comments function to retrive the comment count.Pass the user id of perticuler user as an argument.

$args = array(
    'user_id' => 1, // use user id
    'count' => true //return only the count
    );
$comments = get_comments($args);
echo $comments

Please refer below link for more information.
http://codex.wordpress.org/Function_Reference/get_comments