I hope you don't have the intention to "hack" Magento core files. I recommend to create your own classes / interfaces for that! 

Anyway if you are implementing a custom method `updateStockItems` for a webapi which updates more stockitems with a single request you should take care of the right method signature: You are expecting an array of stock items, therefore your input parameter should be of that type.

Your interface:

    /**
     * 
     * @param \Magento\CatalogInventory\Api\Data\StockItemInterface[] $stockItems
     * @return int
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function updateStockItems(\Magento\CatalogInventory\Api\Data\StockItemInterface[] $stockItems);

Your method implementation:

    /**
     * 
     * @param \Magento\CatalogInventory\Api\Data\StockItemInterface[] $stockItems
     * @return int
     * @throws \Magento\Framework\Exception\NoSuchEntityException
     */
    public function updateStockItems(\Magento\CatalogInventory\Api\Data\StockItemInterface[] $stockItems)
	{
		foreach ($stockItems as $stockItem){
			//do whatever with your stock item
		}
	};

Please take care to use only parameter in your JSON objects which match the used interfaces, in this example `\Magento\CatalogInventory\Api\Data\StockItemInterface`. So putting there some key `sku` doesn't make sense and should either be ignored by Magento or give you an error in the API call.

Here is the documentation about the usage of custom webservices.

https://devdocs.magento.com/guides/v2.3/extension-dev-guide/service-contracts/service-to-web-service.html