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

validate_product_message.php

<?php

/**
 * Validation function to limit the quantity.
 *
 * @param int    $product_id  Product ID.
 * @param int    $quantity    Product quantity.
 * @param string $action_type Type of the action. Allowed 'add' or 'update'.
 *
 * @return bool Return true if quantity is not exceeded else false.
 */
function validate_product_qty( $product_id, $quantity, $action_type ) {
	if ( is_discounted_prdct_qty_exceeded( $product_id, $quantity, $action_type ) ) {

		$output_string = wp_sprintf(
			'%s %d.',
			__( 'Discounted product maximum quantity is ' ),
			MAX_QUANTITY
		);

		wc_add_notice( $output_string, 'notice' );
		return false;
	}

	return true;
}