To enter the serial numbers of the products in a document, just enter the product's SKU and an additional box will pop up. Here you can add the serial numbers of the products you are about to receive. You can set first the Quantity of the products and then add the serial numbers.
If you type less serial numbers than the Quantity you have entered earlier and try to verify the document, the system will alert you that you have not added all the serial numbers. Alternatively, first add each serial number and the quantity will increase automatically.
Also, in the Serial Numbers tab you can see all the Serial Numbers that appear in a specific document.