You can add as many products as you like! In fact you can add as many products, custom product and surcharges to an existing order! The only limitation is that you can only add one addition order to an existing order.
So for example if someone places an order and then you add a sub order to it with a few more items and they process the payment for it, you can't then add another. You can however create a new order and merge it after it's been placed :-)