Not to belabor the point, but any number of lanes will end up with one or two that are "fastest" and racers will gravitate toward them. If you have 4 or 6 lanes, then it's usually the center two that are best, as they will never have the inside of the sharpest turns, which often trip people up.
But you're right, with 20 cars, more than two lanes would be actually useful. Of course, at that point, you're no longer needing/using the original lap counting system, and you'll have to re-do the track anyway for the new system, so the point is moot. ;-)