I just ordered 5 a couple of days ago without a problem. They each shipped separately (each with their own FedEx tracking #), all direct from Shenzhen CN, but nonetheless were on the same order and charged as one charge to my visa.
For experimentation purposes I just went back and tried to order 6. When I tried to set the quantity to 6, I received a message saying that iPhone orders are limited to 5.
So I guess the answer is a max of 5 per transaction online, and 2 per transaction in stores.