The game will launch on a console with a small install base, which is obviously a big negative for sales.  But, Nintendo games often have very high attach rates, the game is highly anticipated, and the options for other games for NS owners are pretty limited.  So, I don't know what to expect from Splatoon.  I'm sure it will have great legs.  But, will it really be selling well come holiday 2018, or after that, when its not the hot, new game?

3-4 million only in Japan and 3-4 Million NA, Europe and Rest of the World.

Shit. I voted 4-5 million thinking only of Japan.

Japan    3    million

NA         4    million

Others  2.5  million

Total     8.5 million 

The current installbase is not a negative to lifetime sales. That didn't stop MK7 and SM3DL from selling more lifetime than there were 3DS owners when they launched. The installbase will grow and legs are a thing which the first Splatoon showed the series very much has.

Also can't vote in your poll because 10+ million isn't an option. The way I see it, if other Wii U games that sold similarly to Splatoon had their 3DS installments break 10 million on a successful system (3D n 2D Mario), then Splatoon 2 should definitely do similarly on an even more successful system.

The small current install base does not necessarily mean it won't sell a ton in the long run.  But, games sell best when they're new, and any game is limited by the number of people that own the hardware.  So, the current install base is most definitely a negative.  Its just the degree to which it holds down sales that is unkown.  

6-7 million
10+ million Splatoon 3

Well over 10 million, hell, maybe even over 15 million.
And there are a few reasons as to why I think this.
1. Splatoon 1 managed to nearly reach over 5 million on the Wii U, something that is VERY impressive for an new IP that many people were unsure of at first (There were those people who thought it would sell leas than Wonderful 101.) Imagine what it can do on a system that sell is selling at thenlevels the Switch is selling right now? Especially now that it has a strong fan base and strong momentum behind it?
2. Japan is all about portabilitity. Splatoon 2 will be on a system that effectively and eventually replace the 3DS as the number 1 dedicated gaming system in that country. Out of the 5 million copies the first game did, around 1.6 was in Japan, where the Wii U's install base is less considerably less than North America's. 2 will end up in one of these three levels over there:
Level 1: Pokemon/Animal Crossing
Level 2: Monster Hunter/Dragon Quest
Level 3: Mario Kart/2D Mario
Personally, I think it'll end up in level 1. So around 4.5-5 million in Japan, plus 5 million in NA. 3.5 million in Europe atound 800k-1 million in the Rest of the World to round it up to around 14-15 million lifetime.
The hype is real! The momentum is real!
Mario, Pokemon, Zelda, and Animal Crossing, make room!!
You've got one more joining you to make a full starting lineup of All Stars that could dismantle the Warriors AND Cavaliers!
Nintendo's Big 5!!

Splatoon sold around 4 million outside of Japan

I wanna say it can sell over 10 million, but I'm not confident enough. Let's go with 9 million then!

my prediction is 6m-8m. its hard to predict Splatoon after Splatoon 1 (my prediction for Splatoon 1 was 1m-1.5m)