According to Industry analyst Daniel Ahmad, the Xbox One has surpassed 41 million sales, which is a decently impressive number in a vacuum, but compared to the Xbox 360's sales last generation and the PS4's sales this generation, it's not a great look for the Xbox brand.
According to Ahmad, in 2018, hardware was actually up year-on-year for Xbox, thanks to the release of Xbox One X. Further, software numbers for the system remain healthy. In fact, according to Ahmad, the digital shares on Xbox One compared to other platforms is pretty substantial.
Of course, the 41 million figure and the finer details around it should be taken with a grain of salt. Ahmad is known for his industry insight and is privy to a lot of information that many of us aren't, but unless the numbers are coming straight from the mouth of Microsoft, it's always best to remain at least a little bit skeptical.
It's been widely known for awhile that the Xbox One has been trailing the PS4 2:1 for most of the generation though, and it seems like as this console generation winds down, the gap has started to increase.
As you may know, as of December 2018, the PS4 had surpassed 91 million units, which is nine million more than double of Xbox One's numbers, with a month to spare. In other words, while the PS4 looks very likely to easily clear 100 million by the end of the generation, the best the Xbox One can probably hope for is 50-55 million.
Removing PlayStation from the equation, this generation surely has been a dip for Xbox, who sold over 84 million Xbox 360s as of June 2014. However, when you compare the Xbox One's numbers to the original Xbox -- which sold 24 million plus -- it doesn't look as bad. However, it's worth noting that the Xbox was Microsoft's debut console and had a much shorter lifespan than most consoles.
Anyway, at the end of the day we still don't have concrete Xbox One sales numbers, and we likely won't ever. But from the sounds of it, the 2:1 ratio has been a pretty steady ratio, one that Xbox is surely eager to turn around with the new console generation approaching.