3DS: 75 Million
PS4: 110 Million
XB1: 50 Million
Switch: 50 Million
3DS still sells about five million a year and should have three more years of console production. I think the third year should slow down a ton, but that's easily another 10 million units by the time Nintendo finally pulls the plug.
PS4: It's at 62 million so far, and still sells around 100,000-200,000 units a week. The system still has five to six years before Sony finally stops production. Keep in mind that both the PS3 and PS2 were still being made years after their successors launched.
XB1: MS has given up but they still match 3DS sales. Over the next four years they should be able to stick to 5 million a year or so. I don't see the XB1 staying in production for as long as the other systems though. MS is retreating from the console market. XB1X is their last console model.
Switch: I think it will have a life cycle of five years (like most home-based Nintendo consoles), but unless there is a drastic hardware revision, it won't pass much beyond 50 million units. Keep in mind that Switch is popular with *gamers*, not the general public. There's a core audience of 25 million Nintendo fans most of who skipped the Wii U, and then there's about 25 million general gamers who will pick it up. IMO anything past 60 million Switches sold lifetime would be a miracle. But on the bright side, I think Switch will have a massive software attach rate, and will make Nintendo a ton of money.
The successor to the Switch will be the last man standing in a console apocalypse, where Sony/MS have left the market and every game is $80 on release with a digital only version, and $30 worth of DLC planned a month after release.