3 Million is the most public estimate of number of Alexa access units sold to-date. Link. Seems really small. You're going to have "voice activated assistants" in your home and then want them in your office sooner than later. If it's not an Amazon device it'll be a Google (Google Home is built on Chromecast not Android?) or an Apple device.
BTW, the Echo Dot (plug it into other speakers) is on backorder. Should ship again in June. Get one and learn. The Amazon Tap (a smaller, portable Echo) requires you to tap a button to talk to Alexa. No thanks.