Where is the Purchase order number on the purchase order to use to buy items from my vendor?

The PO number is at the top right of the PO form.

Filling out the purchase order is really just a memo area for your use to remember what was ordered when.  You would have to print it out and send it to the vendor to place the actual order

Welcome to the Community, @dale23.

Rustler is correct. However, if you're unable to see the Purchase Order (PO) number at the top right, the Use custom transaction numbers for expenses in the Account and Settings may be turned off. If that's the case, here's how to turn it on:

1. In QuickBooks Online (QBO), go to the Gear icon at the top right to get to the Account and Settings. 
2. Select the Expenses menu at the left pane, then tick the Pencil (edit) icon in the Purchase orders section. 
3. Toggle the Use custom transaction numbers option to turn it on, then hit Save and Done.

For more information, check out these articles:

• How to show transaction numbers.
• Create purchase orders in QuickBooks Online.

Please reach out to me in the comment section below if you have any other issues or concerns, and I'll get back to you right away. I'm more than willing to help. Have an amazing rest of the day!