From the relevant approved purchase order's detail page, click Receive Purchase Order.
Receive the order as normal with the correct quantities actually received. Do not enter invoice information, even if you have the supplier invoice.
This will adjust in the exact number received. If you have the supplier invoice, click Update Invoice Details from the order details page and cost the order as per the supplier invoice. Eg, you may have received 15 units, but only invoiced for the original 10. This will adjust the 15 into stock with the landed price being the average of the value of 10.
if you want to receive the extra in as $0, when receiving the purchase order, fully receive the original order qty, and click Add Product to add an additional line item for the same SKU for the extras quantity. Ensure that the additional line's invoice price is $0. This will adjust the original quantity in at the purchased amount, and the extra's as $0. No average pricing.
If you are not planning on keeping the extra quantity, you would usually adjust in what you have been invoiced for, and raising a supplier credit note to return the extras.