Restrict options when variants represent quantity

booking-form.liquid

    // When a date/time is chosen, update the variant option that represents the number 
    // of people attending the event.
    $('form[action="/cart/add"]').on('bta.datetimeChange', function(event, form) {
      var index = form.getAvailability() - 1,
          quantitySelector = $('select[data-option="option1"]', $(this)); // assumes the variant option quantity selector is option1

      // clear any existing disabled options
      quantitySelector.find('option').removeAttr('disabled');
      
      // disabled options > available
      quantitySelector.find(' option:gt(' + index + ')').attr('disabled', 'disabled');
      
      // select first available
      if (quantitySelector.find('option:first').not(':disabled').length > 0 && quantitySelector.find('option:selected').is(':disabled')) {
        quantitySelector.find('option:first').not(':disabled').prop('selected', true)
      }
    });