How to display the number of comments in WordPress

To display the total number of comments, Trackbacks, and Pingbacks for a post, use comments_popup_link or comments_number tags.

These tag must be within The Loop.

In index.php or archive.php

Use comments_popup_link. Will not work if in a page or single post.

<?php comments_popup_link('No Comments', '1 Comment', '% Comments');  ?>

In single.php or page.php

Use comments_number.

<?php comments_number('No Comments', 'One Comment', '% Comments');  ?>