Display count of datetime registrations (sold + reserved) on event listings where datetimes are shown

my_add_sold_of_reg_limit_message.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_sold_of_reg_limit_message', 
  10, 
  2
);
function my_add_sold_of_reg_limit_message( $html, $datetime ) {
  $html .= '<br><span class="space-remain">';
  $html .= $datetime->sold_and_reserved();
  $html .= ' of ';
  $html .= $datetime->reg_limit();
  $html .= ' attendees total</span>';
  return $html;
}

my_event_registration_count.php

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

add_action( 'AHEE_event_details_after_the_content', 'my_event_registration_count' );
function my_event_registration_count( $post ) {
  $event_obj = $post->EE_Event;
  $html = '';
  if ( $event_obj instanceof EE_Event ){
    $reg_count = EEM_Event::instance()->count_related(
      $event_obj, 
      'Registration',
      array( 
        array(
          'STS_ID' => array(
            'NOT_IN',
            array(
              EEM_Registration::status_id_cancelled
            )
          )
        )
      )
    );
    $html .= '<strong>';
    $html .= $reg_count;
    $html .= ' of ';
    $html .= $event_obj->total_available_spaces();
    $html .= ' attendees total</strong>';
  }
  echo $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/f706dd0c1d315289c7293b24a5fa9c02