Need help! How to show comment number of one author/user

I’m making my first theme of WP and am looking for ways to show some details of the post author or user (either registered or not).

I know how to show all the comment numbers from one post by using:

<?php comments_number('0', '1', '%' );?>

tag but there’s nowhere to find anything helpful in the codex. Can anyone help? Thanks in advance!