By end of 2015, XB1 sales should be ~11 million rounded in the US, maybe more.
Back in 2013, XB1 USA sales were < 60% of total worldwide sales in 13 countries. Of course, since that, a lot of think are happened, which could increase, or decrease the US share.
What could decrease:
• New countries released in September 2014
• Unofficial promotion in some Europe market (example, since i'm from Italy, on some store XB1 was selling since March 2015 for €349 with two games)
What could increase XB1 share in the US:
• Official XB1 pricedrop US exclusive to $349 back in 2015.
• Holidays sales usually bigger in the US
Since we also should talk about XB1 performance outside US, in Europe, UK should easy be > 2 million, even if is not confirmed i seriusly doubt is still under those numbers, most likely MS just asked to not annunce this, and in my opinion, XB1 is well over 2m in UK, but that's just speculation, let's not talk about random numbers.
France was ~420,000 by end of 2014, with ~290,000 LT, France should be the second bigger XB1 market in Europe, while Germany was ~378,000 back in March 2015.
Spain is seriusly a death market for XB1, ~78,000 by June 2015, and in Italy we know Wii U + XB1 is < 30% with PS4 >70% total share back in April 2015.
Outside Europe, a rumor has XB1 China sales were at 71,000 back in April. Even if is not confirmed, i think we can use it. Now, PS4+XB1 China sales are ~500,000, we don't know XB1 sales but PS4 has shipped over 400k.
And well, 60,000 in Japan (LOL).
In conclusion, we have no idea of what are the XB1 sales worldwide, so, those 18 million could be true, or not. We just can estimate, all we know is that XB1 is over the 360 launch allinied.
EDIT: How, about Italy, PS4 sales should be >500,000 by end of 2014. But keep in mind that is not official. This can help to know XB1 italy sales since we know PS4 was >70% at one point.