Example of how to pull the required tickets on an event, combine the remaining tickets values and return.

tw_ee_show_remaining_tickets.php

<?php // Please do not include the opneing PHP tag if you already have one.

function tw_ee_show_remaining_tickets($atts) {
	$EVT_ID = get_the_ID();
	$event = EEM_Event::instance()->get_one_by_ID($EVT_ID);
	if( $event instanceof EE_Event ) {
		$required_tickets = $event->tickets(
			array(
				array(
					'TKT_required' => true
				)
			)
		);
		if($required_tickets) {
			// Loop over all of the required tickets for the event.
			foreach($required_tickets as $required_ticket) {
				// Sanity check to make sure we have an EE_Ticket.
				if($required_ticket instanceof EE_Ticket) {
					// Add the spaces remaining for each required ticket to $spaces_remaining.
					$remaining_spaces += $required_ticket->remaining();
				}
			}
		}

		return $remaining_spaces;
	} 
}
add_shortcode('SHOW_REMAINING_TICKETS','tw_ee_show_remaining_tickets');