How can I count only guests comments?

by

You can use these inside the loop, or in comments.php

//The author of current post
$author_ID = get_the_author_meta("ID");
//The current post ID
$p_ID = get_the_ID();
//Number of guest comments
echo count(get_comments(array(
                'post_id' => $p_ID,
                'author__not_in' => array($author_ID)
            )));

More: https://codex.wordpress.org/Function_Reference/get_comments