First example: Show home many tickets have sold of total available below dates. Second example: Show home many tickets have sold of total available below the ticket selector.

ee_number_tickets_after_dates.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_event_date', 'ee_print_number_of_tickets_after_dates', 10 );
function ee_print_number_of_tickets_after_dates( $post ) {
  $event = $post->EE_Event;
  if ( $event instanceof EE_Event ) {
    if ( ! $event->is_sold_out() && $event->is_upcoming() ) {
      //get total approved registrations count
      $spots_taken = EEM_Registration::instance()->count(array(
        array(
          'EVT_ID' => $event->ID(),
          'STS_ID' => EEM_Registration::status_id_approved,
        ),
      ), 'REG_ID', true);
      $html = '<div class="total-tickets">';
      $html .= $spots_taken;
      $html .= ' sold of ';
      $html .= $event->total_available_spaces();
      $html .= ' total available</div>';
      echo $html;
    }
  }
}

ee_print_number_of_tickets.php

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

add_action( 'AHEE__ticket_selector_chart__template__after_ticket_selector', 'ee_print_number_of_tickets', 10, 2 );
function ee_print_number_of_tickets( $EVT_ID, $event ) {
  if ( $event instanceof EE_Event ) {
    if ( ! $event->is_sold_out() && $event->is_upcoming() ) {
      //get total approved registrations count
      $spots_taken = EEM_Registration::instance()->count(array(
        array(
          'EVT_ID' => $EVT_ID,
          'STS_ID' => EEM_Registration::status_id_approved,
        ),
      ), 'REG_ID', true);
      $html = '<div class="total-tickets">';
      $html .= $spots_taken;
      $html .= ' sold of ';
      $html .= $event->total_available_spaces();
      $html .= ' total available</div>';
      echo $html;
    }
  }
}