Magento 2 Cart – How to Get Quantity of Added Product

For checkout_cart_add_product_complete event

<?php
namespace Vendor\Module\Observer;

use Magento\Framework\Event\Observer as EventObserver;
use Magento\Framework\Event\ObserverInterface;
use Magento\Checkout\Model\Session as CheckoutSession;

class CheckoutCartAddProductCompleteObserver implements ObserverInterface
{
    private $_checkoutSession;

    public function __construct(
        CheckoutSession $checkoutSession
    )
    {
        $this->_checkoutSession = $checkoutSession;
    }

    public function execute(EventObserver $observer)
    {
        /** @var \Magento\Catalog\Model\Product $product */
        $product = $observer->getEvent()->getDataByKey('product');

        /** @var \Magento\Quote\Model\Quote\Item $item */
        $item = $this->_checkoutSession->getQuote()->getItemByProduct($product);

        $itemId = $item->getId();
    }
}

Its because of Qty and Special Price both are external attributes of product,so if you directly called via $item then you will not get the value for the same

Try below code :

You can get Qty or other values like min qty by using that method.

echo $item->getExtensionAttributes()->getStockItem()->getQty();

OR Complete Object Data

print_r($item->getExtensionAttributes()->getStockItem()->getData());

clear the cache and check it will works.

Update :

Load stock interface , try below code :

 public function __construct(
    \Magento\CatalogInventory\Api\StockStateInterface $stockItem
   )
  {
    $this->stockItem = $stockItem;
  }

 $this->stockItem->getStockQty($product->getId(), $product->getStore()->getWebsiteId());