Most of my projects tend to come in between 12 and 20 tracks, as a rule, after comping, and sometimes with some bouncing. I'm overdue building a new desktop, so if I get much over 20 tracks on my 500 mHz Pentium 3, I start losing the ability to use all the plug-ins I want. (44.1 / 24 bit)
My new Centrino laptop seems to promise considerably more horsepower, but I haven't given it and its companion MOTU 828 that many hard workouts, yet. It was fat and happy dropping in 10 new tracks at a time, though.
Interestingly, comparing ACID Pro 3 with a friend's new Garage Band, I was able to add 20 stereo tracks to a 12 loop project -- on the P3! 16 bit and no plug-ins, though...
PS... Yeah, messyman, s'up with that 48 kHz stuff? You doing work for video?