Nintendo 3DS: 74 M
PlayStation 4: 130 M
Xbox One: 60 M
Nintendo Switch: 90 M

I have no idea. I just hope not too much so they have the motivation to improve their current model.

3DS: 72m
XB1: 57m
PS4: 114m
Switch: 100m

P.S. Though xb1 and switch very hard to predict. In the case of Xbox we don’t know yet if Microsoft will release the next console after one x as another Xbox One family device or not so my prediction only counts for original one, One s and one x consoles. In the case of switch it is hard to predict it’s lifetime. If it’s almost 100% known that 3ds support will be dropped next year or in the best case in 2019 which I don’t really see happening and PS4/xb1 lifetime will end in 2020-2021, it’s hard to say when the switch will be dropped.

3DS: 77m
PS4: 120m
Xbox One: 48m
NS: 85m

If we will see Switch Pro and Switch Mini, then sales maybe reach 100 mln.

PS4: 113M
XBO: 53M
NS: 96M
3DS: 75M

3ds: 77 m
Xbox one: 52m
NS: 84m
PS4: 123m

PS4: 135 M
3DS: 72 M
Xbox One: 61 M
Switch: 85 M

3DS: 69M
PS4: 105M
Xbox One: 45M
Nintendo Switch: 100M

PS4: 120M
NS: 85M  (early optimistic prediction)
XOne: 55M
3DS: 77M 

PS4: 100 Million
3DS: 75 Million
Switch: 70 Million
Xbox One: 55 Million