Just out of curiosity, what would be an appropriate amount of content?
For comparison's sake, Sonic and Sega All-Stars Racing had 24 tracks (all new since it's the first one). It's sequel, Sonic and All-Stars Racing Transformed, had 21 tracks, 17 of which were new (one was DLC). The former has eight more original tracks but eight fewer overall. The latter had a comparative amount of new tracks but added boats and planes with tracks that changed mid-race. Mario Kart 8 seems comparable to that. Of course, Gran Turismo 6 had like 70 tracks, but they're different kinds of racing games so I'm not sure comparisons are especially useful or fair here.
It's difficult for me personally to comment on what justifies $60 worth of content since I haven't purchased a $60 game in some time. I still haven't bought Mario Kart 8 and I suppose when I do, I won't be spending $60 since I have some trade credit. As a point of reference, I played and enjoyed the ever-repetitive **** out of Mario Kart 64, and my mom spent $50 on it (which is almost $74 today). I wouldn't say no to more content because I love value. However, taking into account what we used to get, rising development costs, and comparisons to similar titles, Mario Kart 8 doesn't seem too light on content. We don't know if Nintendo plans on offering DLC in the future (besides that random free Mercedes-Benz kart" in Japan), but they have given DLC away for free and their paid DLC has been fairly substantial.