comments_number
comments_number is a filter on the text returned by the comments_number() function. Note that for filtering the actual number returned, you should use the get_comments_number hook.
Context:
File: wp-includes/comment-functions.php
function comments_number( $zero = 'No Comments', $one = '1 Comment', $more = '% Comments', $number = '' ) {
global $id, $comment;
$number = get_comments_number( $id );
if ($number == 0) {
$blah = $zero;
} elseif ($number == 1) {
$blah = $one;
} elseif ($number > 1) {
$blah = str_replace('%', $number, $more);
}
echo apply_filters('comments_number', $blah);
}
This hook is a filter which means that information is passed through it, and then used by WordPress. Your function needs to accept that information, and return it. Using add_filter('comments_number', 'your_function'); helps you to remember this distinction. When you are passing an ID, it is assumed that you will return the ID as it was given to you. With filters that pass strings or arrays, you may manipulate the information before passing it along.