example.php

<?php

function ee_print_number_of_tickets_shortcode( $attributes ) {
  $event = null;

  if( !empty( $attributes['event_id'] ) ) {
  	$event = EEM_Event::instance()->get_one_by_ID($attributes['event_id']);
  }

  if( !$event instanceof EE_Event ){
  	$event = EEH_Event_View::get_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>';
      return $html;
    }
  }
}
add_shortcode( 'EE_NUMBER_OF_TICKETS', 'ee_print_number_of_tickets_shortcode' );