The market research group Above Avalon has studied the fortunes of the Apple Watch series up to 2020, and has put the number of its adopters at 100 million. This would make it the OEM's fourth biggest product, with iPhones, iPads and Macs the only lines to sell better than these wearables. Should this trend continue, they may in fact be able to overtake the PCs by 2022.
https://www.notebookcheck.net/There-are-now-100-million-Apple-Watch-users-on-Earth-new-report.520129.0.html