I paired 3 watches to one iPhone. I don't have more to test how many you can pair.
It is the single BEST feature I was waiting for after watch OS 2 came out. Before, I had to unpair them and pair them every morning. Now, it is just awesome. I just choose what I am going to wear, then pick my Apple Watch and a band, just like a regular watch.
At first, I thought it would detect them by putting them on and then unlocking them, but no. If you are not wearing them, it automatically switches to the one you are moving. So I guess the motion coprocessor is always on. In the photo, the last watch selected is the last one I put on the table.
A downside of this is that they run out of battery sooner before this feature was released. Before, just not wearing them, the battery would last at least a MONTH. Now, since it is always on, battery doesn't last even a week. I guess I'll put them all on chargers.
It is awesome.
What happens with the workout data - do they all show the same data - so that (say) if you worked out (swimming) for 70 minutes under "workout other" and stood for 6 hours, would the 2nd watch you put on show the data so that you'd only have 6 more hours to do for the "stand goal."