Show comment number per author per day

You can try this query that counts user comments by using the user_id field in the comments table as a filter:

function count_user_comments_today( $uid ){
    global $wpdb;
    $today = date('Y-m-d');
    $tomorrow = date('Y-m-d', time() + 86400);
    $count = $wpdb->get_var( $wpdb->prepare("SELECT COUNT(*) FROM {$wpdb->comments} WHERE user_id = %d  AND comment_date >= %s  AND comment_date < %s ", $uid, $today, $tomorrow  ));
    return $count;
}

where we use PHP to give the current date and we don’t use any SQL date functions on each row.

You could also consider using the WP_Comment_Query class if you are looking for a more native solution.

Usage:

You can use it like this for the current logged in user:

global $current_user;
get_currentuserinfo();
echo count_user_comments_today( $current_user->ID );

and for the current author:

echo count_user_comments_today( get_the_author_meta('ID') );

in the loop in the template page author.php.

Outside the loop in author.php you can use:

global $wp_query;
$curauth = $wp_query->get_queried_object();
echo count_user_comments_today( $curauth->ID );

for the current author.