tribe-max-tickets-allowed.php

<?php

function tec_max_tickets_allowed() {
  if ( ! function_exists( 'tribe_is_event' ) || ! tribe_is_event() ) {
		return false;
	}
  ?>
  <script>
    jQuery( function( $ ){
      if( $(".tribe-ticket-quantity").length ){
        $(".tribe-ticket-quantity").attr("max", 1);
      }
    } );
  </script>
  <?php
}

add_action("wp_footer", "tec_max_tickets_allowed");