Over 1.3 Billion iOS devices have been sold since the first iPhones were sold on June 29, 2007. over 450,000,000 of those in the last four years alone were iPhones capable of connecting to the Apple Watch. Apple claims that 700-800 million iPhones are still active around the world. The balance of the iOS devices are iPads and iPods (not all iPods, especially the early models, ran on iOS).
“450,000,000 of those in the last four years alone were iPhones capable of connecting to the Apple Watch”
That means so far less than 1% of those who can use the watch wanted to order it.