/**
 * Set an order quantity multiple for checkout
 */

add_action('woocommerce_checkout_process', 'cc_order_quantity_multiple');
add_action('woocommerce_before_cart', 'cc_order_quantity_multiple');

function cc_order_quantity_multiple() {

  // Set this variable to specify an order quantity multiple
  $multiple = 6;

  if ((WC()->cart->get_cart_contents_count() % $multiple) > 0) {

    if(is_cart()) {

      remove_action('woocommerce_proceed_to_checkout','woocommerce_button_proceed_to_checkout', 20);
      wc_print_notice(sprintf('You must place an order with a multiple of 6 items'), 'error');

    } else {

      remove_action('woocommerce_proceed_to_checkout','woocommerce_button_proceed_to_checkout', 20);
      wc_add_notice(sprintf('You must place an order with a multiple of 6 items'), 'error');

    }
  }
}