Set a maximum quantity for a product in WooCommerce

max-quantities-woocommerce.php

<?php // only add this line if it's at the top of a new PHP file

add_filter( 'woocommerce_add_to_cart_validation', 'validate_add_to_cart_action', 10, 3 );

function validate_add_to_cart_action( $valid, $product_id, $quantity ) {
	// Set the product ID for the product you want to limit
	$check_product_id = 121;
	// We want to check two quantities, the current cart and how many you are currently adding.  We're going to be adding them together into one variable called total_quantity.
	$total_quantity = $quantity;
	$maximum_quantity = 2;
	
	// Setting the default to valid
	$valid = true;

	// Checking if it's the product ID we want to target
	if ( $check_product_id === $product_id ) {
		// Loop through the items in the cart first
		foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
			// If the item in the cart matches our product id, we'll add the current cart quantity to our total quantity
			if ( $values['product_id'] === $check_product_id ) {
				$total_quantity += $values['quantity'];
			}
		}
		// Let's check our total quantity now and see if it's above our maximum - if so, we'll display an error message
		if ( $total_quantity > 0 && $total_quantity > $maximum_quantity ) {
			// Grab the product object so we can output which product has the issue
			$_product = wc_get_product( $product_id );

			$message = sprintf( 'You can only have a maximum quantity of %d for the %s product (you currently have %d in your cart).', $maximum_quantity, $_product->get_title(), $total_quantity - $quantity );

			wc_add_notice( $message, 'error' );
	
			$valid = false;
		}
	}
	return $valid;
}

add_action( 'woocommerce_check_cart_items', 'set_max_for_cart_item' );

function set_max_for_cart_item() {
	// Set the product ID for the product you want to limit
	$check_product_id = 121;
	// Set the maximum quantity
	$maximum_quantity = 2;

	// Loop through the Cart items
	foreach ( WC()->cart->get_cart() as $key => $values ) {
		$_product = $values['data'];
		// Check for the speicifc product
		if ( $check_product_id === $values['product_id'] && $values['quantity'] > $maximum_quantity ) {	
			// If we match the product, we set an error message
			$message = sprintf( __( 'You can only have a maximum of %d for the product "%s".' ), $maximum_quantity, $_product->get_title() );

			wc_add_notice( $message, 'error' );
			return;
		}
	}
}