add_action('woocommerce_checkout_process', 'quantity_checkout_field_process');

function quantity_checkout_field_process() {
    // Check if set, if its not set add an error.

    //Check for product quantities
    $quantity_of_products_in_cart = product_quantities_in_cart();

    if($quantity_of_products_in_cart != 0) {
        $i = 0;
        while ($i < $quantity_of_products_in_cart) {
            $j = $i + 1;

            $attendee_name = 'attendee_name'. $j;
            $attendee_last_name = 'attendee_last_name'. $j;
            $attendee_phone = 'attendee_phone'. $j;
            $attendee_email = 'attendee_email'. $j;

            if ( ! $_POST[$attendee_name] )
                wc_add_notice( __( 'Please check on filling the First Name for attendee '. $j .'.' ), 'error' );

            if ( ! $_POST[$attendee_last_name] )
                wc_add_notice( __( 'Please check on filling the Last Name for attendee '. $j .'.' ), 'error' );

            if ( ! $_POST[$attendee_phone] )
                wc_add_notice( __( 'Please check on filling the Phone for attendee '. $j .'.' ), 'error' );

            if ( ! $_POST[$attendee_email] )
                wc_add_notice( __( 'Please check on filling the Email for attendee '. $j .'.' ), 'error' );

            $i++;

        }

    }

}