It appears you can buy as many as you like, however I couldn't checkout as I'm not based in the US anyway, so it's possible down the line an error message might pop up. If you need more than two, try it out, see what happens.
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.