Add a product and quantity to cart, based on another product and quantity.

wc-add-product-to-cart-based-on-another-product

add_action( 'woocommerce_add_to_cart', 'prefix_add_additional_product', 10, 6 );
/**
 * Adds additional product to cart based on the quantity of an added product
 */
function prefix_add_additional_product( $cart_item_key, $product_id, $quantity, $variation_id, $variation, $cart_item_data ) {
	// Get the cart
	$cart = WC()->cart->get_cart();

	// Nothing, if the cart is empty
	if ( 0 == count( $cart ) ) {
		return;
	}

	// Enter the free product ID
	$additional_product_id = 96;

	// Don't add accessory when adding the same accessory
	$added_product_id = 0 < $variation_id ? $variation_id : $product_id;
	if ( $added_product_id == $additional_product_id ) {
		return;
	}

	// You can add checks for specific product ID or some other requirements here

	$additional_product_quantity = (int) ( $quantity / 5 );

	WC()->cart->add_to_cart( $additional_product_id, $additional_product_quantity );
}