How do I print a single purchase order per page?

There are many ways to accomplish your goal. For instance, you could put the loop in your print program itself, wrapped around the successive calls to the Smart Form, passing in the changed document information each time.

However, if I understand your requirement correctly, I would recommend that you write your print program to produce a single PO at a time and leverage 'wrapper' transaction ME9F (PO message output) to handle the looping for you. One of the benefits of this approach is that it ensures the message control table NAST gets updated properly when output is issued for each individual PO.