This gist is used in the tutorial https://www.vijayan.in/how-to-limit-the-cart-quantity-of-discounted-product/

add_to_cart_validation.php

<?php
define( 'MAX_QUANTITY', 3 );

add_filter(
	'woocommerce_add_to_cart_validation',
	'validate_discounted_prdct_qty_limitation',
	10,
	3
);
/**
 * Hooked functionality to limit the discounted product quantity to checkout.
 *
 * @param bool $passed      Validation.
 * @param int  $product_id  Product ID.
 * @param int  $quantity    Product quantity.
 *
 * @return bool Return true if quantity is not exceeded else false.
 */
function validate_discounted_prdct_qty_limitation( $passed, $product_id, $quantity ) {
	$product_object = wc_get_product( $product_id );

	$parent_id = $product_object->get_parent_id();
	if ( ! empty( $parent_id ) ) {
		$product_id = $parent_id;
	}

	return validate_product_qty( $product_id, $quantity, 'add' );
}