Hi Rob,

Let's make sure you are creating purchase order in the Item details section to be able to add QTY in your PO. If you are in the Category details section QTY is not available.

To create a purchase order:

1. Select + New.
2. Select Purchase order.
3. Select the Supplier name.
4. Under Item details select the Product/Service, QTY, rate, GST and etc.
5. Click on Print to view the details on the form.
6. Click on Save and send to send the PO to your supplier.

Check this link to learn more about creating purchase order in QuickBooks Online. 

Drop by again in the Community if you have further questions about PO In QBO. I'm around to help you.