Joe Rossignol, Macrumors:
The latest data from Strategy Analytics reveals that the Apple Watch remained the most popular smartwatch through the fourth quarter of 2015, capturing 63 percent global market share based on an estimated 5.1 million sales in the three-month period. […]
Strategy Analytics previously estimated Apple Watch sales at 4 million in the second quarter, and 4.5 million in the third quarter, meaning that Apple sold an estimated 13.6 million Apple Watch units overall in 2015.
It’s important to note that these are estimates — Apple is not yet reporting sales of the Watch. Furthermore, Tim Cook earlier this month said only that holiday quarter Watch sales exceeded that of the original iPhone. I interpreted this to mean that sales fell somewhere between that of the original iPhone and the 3G; otherwise, it seems likely to me that he would have used the latter figure.
Nevertheless, over 13 million Apple Watches shipped in the first three quarters is nothing to sneeze at, especially for a pricey accessory. For comparison, it took the iPhone over twice as long to sell as many.