They should make it and a crash kart game at the same time. My wife would want a ps4 just for either. 

I could see 2.5m happening, Crash just isn't as popular nowadays like he used to be

ND already said they wouldn't develop the game. Heck, they didn't develop Jak 4 and chose to go with TLOU instead (and I'm glad they did). ND will develop cinematic, story driven games going forward.

If Sony bought the IP, then the best developer/combo for it would be SCEJA with ND overseeing it IMHO. Crash is one of the very few western franchises that succeeded in Japan, hence why I think it would be a good combo. However, buying the IP is not that simple, Crash hasn't scored in the high 80s/low 90s since Warped (i.e. 1998). There hasn't been a mainline Crash game since Mind over Mutant (2008). The last Crash game was released on iOS in 2010. Millennial generation don't know who he is. Outside of the internet, people don't know who heis. The series has been irrelevant, both critically and sales wise, since Wrath of Cortex (2001) on PS2. It is a VERY high risk/high reward investment. How much would Acti want for a franchise that has sucked over the past 15+ years and has not had high enough sales in the meantime, but has huge potential if used right? How much would Sony be willing to pay and still be able to regain its initial investment? $1 million? $10 million? $50 million? $100 million? What is a good and fair price on the IP?

To answer the OP's question, depending on market, PS4 marketshare, game's quality, etc, it could go from ~1M to 10M+.

Some of you guys don't understand how Crash was in the PS1. It is not an exaggeration to say that Crash was neck and neck with Mario in WW sales. Everything went downhill after ND lost the license though and it would definitely be hard to regain its success. A complete reboot would be the way to go with that and can prove successful. Tomb Raider is a good example of that.

If Acti doesn't sell the IP, then I don't know, depends how they treat the franchise, I guess, but given that Spyro is already declining, I don't see how they would try to revive Crash to be honest.

2-2.5 million seems to be a safe bet.

If it's Naughty Dog who make it 5m-6m

i think 2M tops

Does this new generation of kids even know what Crash is?

How do you know only kids like Crash

I'm going with 6 million ( That's the minimum sales most Naughty dog games reach nowadays.)

Knack is a very similar style of game to Crash, it was a new IP and sold a full million, Crash has a well established brand, it's classic, I think it could sell over 5-6M lifetime, it depends entirely on what the developer does with the series and who makes it, in this hypothetical scenario.

If ND made it then I think it could go much higher than 6 million.