Open the functions.php file that you can find into your template folder, if you haven’t it you have to create it. After this, add this code at the bottom of file:

function comment_count( $count ) {
	if ( ! is_admin() ) {
		global $id;
		$comments_by_type = &separate_comments(get_comments('status=approve&post_id=' . $id));
		return count($comments_by_type['comment']);
	} else {
		return $count;
	}
}
add_filter('get_comments_number', 'comment_count', 0);