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

validation_business_logic.php

<?php
/**
 * Helper function to check discounted product add to cart limit reached.
 *
 * @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 exceeded else false.
 */
function is_discounted_prdct_qty_exceeded( $product_id, $quantity, $action_type ) {

	$product_obj   = wc_get_product( $product_id );
	$regular_price = $product_obj->get_regular_price();
	$sale_price    = $product_obj->get_sale_price();

	$cart_id              = WC()->cart->generate_cart_id( $product_id );
	$find_product_in_cart = WC()->cart->find_product_in_cart( $cart_id );

	if ( ! empty( $sale_price ) &&
		(float) $sale_price < (float) $regular_price ) { // Validate is discounted product.

		if ( 'add' === $action_type &&
			! empty( $find_product_in_cart ) ) {

			$cart_item = wc()->cart->get_cart_item( $cart_id );
			$quantity  = $cart_item['quantity'] + $quantity;
		}

		if ( $quantity > MAX_QUANTITY ) {

			return true;
		}
	}

	return false;
}