Display remaining datetime registrations quantity on event listings where datetimes are shown

add_total_spaces_available.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter(
  'FHEE__espresso_list_of_event_dates__datetime_html',
  'my_add_total_spaces_display', 
  10, 
  2
);
function my_add_total_spaces_display( $html, $datetime ) {
  $html .= '<br><span class="space-remain">Total spaces remaining: ';
  $html .= $datetime->total_tickets_available_at_this_datetime();
  $html .= '</span>';
  return $html;
}

You can add the above to a functions plugin or into your WordPress theme's functions.php file.

see also:

https://gist.github.com/joshfeck/d4ddcd2f2d25732d7a666123ab99a4e5