Really, the benchmark for the iPhone should be the iPad. The iPad is just as useless to the majority of people as an Apple Watch.
The iPad sold around 330k on launch day and 1m by the first month. What percentage of iPhone users bought iPads? Let's assume that almost every iPad owner was already an iPhone owner. There were around 35m iPhones floating around by then, so that's a 1% penetration rate on launch day, and around 2.81% by the first month of availability.
Using that same proportion as a baseline, we can do some simple numbers. There are around 600 million iPhones today. Assuming a 1% penetration of that market, that's 6m apple watches on launch day, and 17m in the first month.
Of course those numbers are wrong since the 600m is sold, not in use. How many iPhone 5-6s are out there? Around 300m, which gives us 3m apple watches on launch day, 8.5m in the first month - if they can make that many.
Given how the preorders are going, they won't have retail stock until July.