Show total quantity of products ordered on invoice

You can have a look at the internal API docs from WooCommerce.
https://docs.woocommerce.com/wc-apidocs/source-class-WC_Order_Item_Product.html#251-259

$total_quantity = 0;
foreach ( $order->get_items() as $item ) {
$total_quantity += (int) $item->get_quantity();
}

Something like this should give you the total quantity of all ordered products. Not tested!