One motor might be enough for a turntable like the one you made (maybe even a smaller motor could have worked the same) but carrying a load on wheels or tracks is a very different scenario!
The main difference I guess would be the load applied on the motor driven axle: in your turntable, given the large number of rollers you built, it's taking really only a fraction of those 12 kg. Also, apart from the friction acting against the rollers, there are very little forces opposing the motor's movement, in a wheeled base you would also have to factor in the friction applied by the ground to the wheels creating another opposing momentum( which would just grow with the radius of the wheel).
Once you get to a noticeable load on a plastic axle "freely" rolling in a plastic shell, given that there are no real bearings in LEGO, just the friction forces between the elements start to act as a very effective brake on the driving axle, in that case more than one motor will have to be used just to get the axle moving..
I guess that if you really want to limit the number of motors used you first have to limit the load applied on the driving axle (or axles) by maybe just adding more wheels to the base. Even then however, I think that for some loads, friction would add up really rapidly no matter how many different points you're using to redistribute the load.
I just wanted to point these few things out, having noticed your interest on this topic in a couple of posts:) @oracid