It would appear that Psychonauts 2 has sold at least 1.7m copies. That number is arrived at by knowing the first Psychonauts sold almost 1.7m copies and then taking taking this Tweet at face value:
This is not my original analysis. It’s from a thread at /r/GamingLeaksAndRumours.
But to that I would add that I don’t think it’s 100% clear cut. It depends a little bit on how you define “best selling.” I think traditionally it is indeed defined by copies sold, but I wouldn’t hold it against anyone to use the Hollywood metric of dollars spent. In which case the combination of inflation, and the fact the the original Psychonauts was included in a Humble Bundle, would mean that the original has sold likely more copies.
But assuming Lisette Titre-Montgomery is indeed referring to copies sold, it’s a hell of an accomplishment. Makes me think odds of another Psychonauts must be quite high.