As of March 31, 2019...

16.69 mil for Mario Kart 8 Deluxe

8.44 mil for Mario Kart 8

That's 25.13 million units.

Both versions are the best-selling title on their respective console. Nintendo really should release Mario Kart 9 by 2022 or sooner. It would sell for sure, especially if it comes out when there aren't many other big games.

So many people think you can't add much more to the series. Give me some new courses (Super Mario Odyssey, hello), new characters, new items, Double Dash mechanic, missions, and I'm all set. You do all those things and market the heck out of the game and it sells 10 million at least. 

Then 1-3 years into the Switch's successor release Mario Kart 10.

The fact that the (possible) best Nintendo console in its entire history does not have its own Mario Kart is quite sad and disappointing. It strikes me that they have not even announced a DLC. Something they have to do, I do not think they're going to leave this game as a simple port.

I had the Wii U with its respective Mario Kart 8. Now I have Switch and I have no plans to buy Mario Kart 8 deluxe unless they announce a DLC. I have to admit that I have been tempted because I love the character of the Inkling Girl but I will wait.

I'm positive Mario Kart 9 will release on Switch. Once the Smash hype dies down next year, Nintendo will need a new big game to sell online subscriptions.

3 years later, and I'm feeling better about this prediction than ever. I'd say MK9 is 90% likely to be a Switch successor launch title at this point, there's just no reason at all for Nintendo to waste time and money resources making it for Switch when MK8D is going to sell big for the rest of Switch's life. It will likely be 2023 or 2024 for Switch 2's launch, so roughly a decade after MK8 originally came out in 2014. In the meantime, I think Nintendo would be wise to reboot F Zero for release before the end of 2021. It would give the Switch a big first party racing title that isn't just a port and help fill the long MK gap. Would also be nice if Nintendo decided to give us a couple new cups worth of courses dlc in MK8D to hold people over.

I feel that Nintendo should make diddy kong racing that way they have a new kart racer to satiate gamers and it gives them a stepping stone toward the design  direction  they might take with MK9.

Yeah there is no reason MK9 will come out before Switch 2. It'll be a Switch 2 launch title in 2023 or 2024. I am surprised they haven't done a new tracks DLC for MK8D. Maybe that will happen at some point, but yeah Switch already has its Mario Kart it'd be very dumb to have a second Mario Kart on the same system.

Switch 2/successor/whatever launch title would make sense.

I still think Diddy Kong Racing is what Retro was working on all those years before they took over MP4 and either they just couldn’t get it right or Nintendo was dissatisfied with it for some reason. For that reason, I don’t expect a Diddy Kong racing game to happen now. And also, it’s a very obscure character to try to name sell a Kart racer to the masses on when you think about it.

They could probably just port MK8 to Switch successor and it will sell just fine.

Having MK9 as a Launch title for Switch 2 will gurantee a successful launch month.