PlayStation 4 family - 115 million
PlayStation 4 has the definitive momentum for this generation, and depending on when they actually end the PS4's life and what kind of price drops we'll see before then, this number could definitely be higher. The success of the Switch doesn't seem to be slowing the momentum of the PS4 down at all, which bodes well for future sales.
Xbox One family - 50 million
The Xbox One isn't doing terribly, but this is a definite downturn in their fortunes when they should be riding the momentum from last generation into better sales this generation. That is Xbox One's biggest failing, being unable to capitalize on the positivity the 360 bought Microsoft. I get the feeling that the Xbox One X will give The XB1 family a much needed boost in sales but will mainly sell to the hardcore Xbox fan and the hardcore graphics enthusiast, but will fail to achieve larger market success among those looking for a new console or those switching or looking for a secondary console; at this point in the generation, the established game library is far more important than graphics horsepower, and while the Xbox One has a very respectable game selection, it is vastly outclassed by its direct competitor, which boasts having the majority of the same 3rd party titles along with some de facto 3rd party exclusives and a superior variety of 1st party offerings (opinions vary of course). I have to reiterate, however, that Xbox One is not selling badly at all, Microsoft is still making a good profit on the system, and I imagine the Xbox team will be taking all the hard lessons they've learned from this generation to heart when they design the successor to the Xbox One. No, I don't think the Xbox is dead; if Sony can learn from the PS3 debacle and get stronger from it, if Nintendo can recover from the WiiU disaster, Microsoft can certainly do the same in the upcoming generation.
Nintendo Switch - 80 million
I want to be hopeful that the Nintendo Switch does even better than 80 million. I'm basing this prediction off of 3DS sales mostly, but they need greater 3rd party support; right now it feels kind of like it's getting leftovers from the other consoles, akin to how the WiiU was at the start. I don't want to base my guess on the mind blowingly fantastic start the Switch has had (two clear GOTY contenders in launch year, wow!), but because Nintendo's 1st party is an impressive juggernaut and 3DS support is winding down, the Switch will undoubtedly be drowning in fantastic 1st party titles in the years to come. Switch has also carved itself a spot as one of the best places to play Indie titles, and Indie developers continue to support the platform without any signs of slowing down. The Switch manages to be a portable console, a respectable main console, and a companion console to more powerful hardware all at the same time, and that gives it huge potential for sales now and into the future.