You can add a purchase order number to a booking by entering the number in the PO field to the bottom right of the booking screen.
If a client provides a different purchase order number for each individual advert, then you will need to add them into MagManager as separate bookings in order to add the PO number to each booking.
It's not possible to add multiple PO numbers to one booking - See Update Below.
The purchase order number is pulled through to the invoice, provided you have the token 'Invoicing Reference' on your Invoice document template
To edit your invoice document template go to System Settings > Under Admin > Select Document Templates > Select System Documents > Select Invoice > from here you can add the token to the document - It can be found in the tokens section on the left side of the editor by going to Invoicing > Reference > Click Save.
With a series of adverts, while you can now change the PO number after each invoice has been raised, and you are ready to raise the next invoice, this would rely on you remembering to update it each month.
On that basis we would still recomend adding, each issue as a separate booking to match the clients individual PO numbers. After all, if the client wanted them all together as one booking they would have supplied just the one PO number.