Depending on what is announced on Switch since we already know most games coming in 2018 for PS4 :

Switch : 22 millions (hard to say, they have been pretty safe with their numbers and i dont see the president of Nintendo randomly expecting impossible numbers).

PS4 : 18.5
I see it slowing down a bit next year. Games have been announced for years and if someone wanted it there was amazing bundles and prices this year. I dont see it going at 150$ next year and if so its not gonna change a lot. It got the pro as a new factor for the whole year (even if it came out in 2016). It got huge japan titles and the Switch was still in heavy stock issues.) Also noting system seller on ps consoles are third party and i dont see next yeat being bigger. RDR isnt as popularas people make it seems. Trailers barely hit 5 million views in months.

Xbox : 7 millions
I dont really care tbh. Huge discounts this year and still no huge titles next year. Pubg is a catastrophe... nah i see this console declining a lot next year since PS4 and Switch will be even stronger in presence.

PS4 20m
Switch 18m
XB1 10m
3DS 3m

NESC/SNESC/N64C 10m

PS4 19 mil

Xbox one 13 mil

Switch 11 mil

Walmart rollback numbers...

PS4 - 22.98 Million
Switch - 17.98 Million
Xbox - 7.98 Million
3DS - 2.98 Million

PS4 - 21 million; Essentially flat YoY.
NS - 20 million, doesn’t even need Pokemon to do it.
Xbox One - 7 million.
3DS - 4 million

Finally someone that has the same Switch prediction as i do. Some of these predictions are too high for the Switch.

yea they act like pokemon is the next coming. when it didnt make 3ds sell over 12 million for the year.

Switch - 18m
PS4 - 18m
XBO - 7m
3DS - 5m

Based on what? 2018 will be the first full year for Switch and Nintendo will more than likely have their stock issues solved. Couple that with consoles historically selling higher numbers in years 2 and 3 than launch year. There's always a chance Nintendo will price cut as well sometime in 2018. What factual evidence, other than bias and anecdotes, do you have to support your claim? 

3ds had its best year, the first year. switch will only get dropped to 249.99 i don't see that causing a huge increase in sales/ also this is just my opinion, i think with such a strong nintendo lineup i think a huge chunk of N fans already picked one up.