If you purchase an iPhone for 600-700 then you are buying it with out contract, which I don't believe there is a limit (except for what's in stock). If you are purchasing with a contact you would at least get the early upgrade price that would be 100 less each on the above prices.
The two I purchased were on a family plan, each purchase under a different phone number but name, cc #, shipping address (obviously) are all the same. So I am not sure if it's by number or by person, but I was able to get 2 to replace the 2 we have.