There is a war raging right now in the voice assistant realm with Apple, Google, Amazon, and Microsoft fighting to have the smartest and most popular voice assistants. A recent study named Google Assistant the most intelligent of the current crop of voice assistants, and they often show up in many places including TVs, PCs, smart watches, and speakers. It's rare for manufacturers of these devices to give up hard numbers on how many devices have sold, but Amazon has stepped up and done just that for Alexa devices. Amazon
That all-inclusive number certainly sheds no light on sales of Echo devices specifically. However, it's a huge number that likely includes devices that aren't Amazon-branded since you can get Alexa on-board smart TVs and other devices. There are over 150 products with Alexa built-in and over 100 of them that shipped in 2018 aren't made by Amazon. Limp declined to give The Verge a specific number on exactly how many Echo Dot devices were sold during the holiday, all we know on that is that Amazon has bragged that it outsold expectations for the device.
Limp did note in his interview that Amazon sold so many Echo Dot devices this Christmas season that it will be out of stock on them through January. He also said Amazon is "pushing pallets of Echo Dots onto 747s and getting them from Hong Kong to here as quickly as we possibly could." Amazon and Google are gearing up for a bit of a rumble at CES 2019 with both having a presence at the show.
Limp reckons there will be multiple players in the voice assistant market for the foreseeable future. Amazon seems to have no fear of competition with Siri, Google Assistant, and other players.