If you include Q4 2015, yes, 500 million compatibe phones is maybe possible in a stretch.
Here are the historic sales in comparison with the obvious spikes in every Q1 (because Apple's FY ends in September with Q4):
http://www.statista.com/statistics/263401/global-apple-iphone-sales-since-3rd-quarter-2007/
The 500 million estimate is really high, it's a stretch for Apple to ship 200 million iPhones per year from now on.
Since we now know the Watch has to be charged daily and there's no killer use case so far (at least I haven't seen one. Why should most consumers want to wear a watch and charge it daily when they have a smartphone in their pocket that does most of the stuff on a bigger screen) I still doubt we see more than 20-25 million in unit sales in year one.
That would equal to an attach rate of about 5%. As I mentioned before, that could drop off later because most people will probably upgrade their iPhone more frequently than the Watch (as long as the Watch isn't stand-alone or subsidized).