Nearly 20m Apple Watches were sold in 2017

"Why is Apple Watch sales momentum growing? My theory is that consumers are starting to see a place for Apple Watch in their lives. While Apple's revised Apple Watch marketing campaign around health and fitness has led to a clearer sales pitch, I think the health and fitness messaging ends up being Apple's way to get its wrist in the door. People aren't buying and using Apple Watch just for its health and fitness monitoring features. There is something more at play here.
Close to 20 million people bought an Apple Watch in 2017 because the device has become a bridge between the present and future. By including a screen, Apple Watch retains the familiarity found with smartphones, tablets, and laptops/desktops. At the same time, Apple Watch is giving wearers a glimpse of the future by introducing new ideas around how artificial intelligence, voice, digital assistants, and smart sensors can come together to produce a new kind of experience. "

Source:  Above Avalon, 23rd January 2018

Note - I can't find an actual source for the '20m' number - but Above Avalon is a reputable source for all things Apple.