I wonder if you’re looking for the wp_count_comments() function.
To retreive comment count statistics for the whole site, you can use:
$comments_count = wp_count_comments();
or for a given post ID:
$comments_count = wp_count_comments( 123 );
It will return an object with the following attributes:
$comments_count->moderated
$comments_count->approved
$comments_count->spam
$comments_count->trash
$comments_count->total_comments
Check the Codex for more information and examples.