The Xbox One X is the latest console from Microsoft and if there is one advantage the console might have over the competition, is that it will be capable of true 4K gaming, as opposed to scaling up from 1080p. Given that 4K is expected to become the new standard in resolution, you could say that purchasing the Xbox One X is almost like futureproofing your gaming.
So much so that according to DFC Intelligence analyst David Cole (via PlayStation Lifestyle), he has predicted that Microsoft could sell as many as 17 million units of the console by 2021. As it stands, the Xbox One X is expected to be released towards the end of 2017, which means that we are looking at a time frame of 3-4 years for Microsoft to potentially hit that mark or go beyond it.
To date Microsoft has yet to confirm how many units of the Xbox One they have sold, but last we heard it was estimated that they might have sold around 18 million units. This was back in 2016 which means that it has been 3 years since it was launched, so it looks like it is possible that the Xbox One X could hit that mark. However we should point out that the Xbox One X is priced higher than the competition at $499 and that could play a factor in how well it does.
It is also estimated that together with the Xbox One sales, the total number of consoles in the Xbox One family of consoles sold could reach 63 million by 2021. As for the competition, the PS4 has sold more than 60 million units, and the Nintendo Switch appears to be doing pretty well too, having recently crossed the 1 million mark in Japan alone.