/**
 * Set a minimum order quantity for checkout
 */

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

function cc_minimum_order_quantity() {

  // Set this variable to specify a minimum order quantity
  $minimum = 12;

  if (WC()->cart->get_cart_contents_count() < $minimum) {

    if(is_cart()) {

      remove_action('woocommerce_proceed_to_checkout','woocommerce_button_proceed_to_checkout', 20);
      wc_print_notice(sprintf('You must have an order with a minimum of 12 items'), 'error');

    } else {

      remove_action('woocommerce_proceed_to_checkout','woocommerce_button_proceed_to_checkout', 20);
      wc_add_notice(sprintf('You must have an order with a minimum of 12 items'), 'error');

    }
  }
}