OS X 10.8.2 max resolution
The max would be dependent on the amount of physical pixels in the LCD screen. You're going to find the limitation will be the display, and not Mac

Considering the 15" Retina MacBook Peo has a 2880 by 1800 which is internally drawn as 5760 by 3600, and it then supports another 2 external Thunderbolt displays. Of course, this is not a theoretical maximum, just a 'practical maximum' using consumer tech.