Four. My dad would line up the tractor at the edge of a field for a straight sweep across such with a disc or whatever, and I would hold the steering wheel straight until I got to the other end of the field (Ford 4N throttle was somehow set for a very slow speed, and it was in ? 2nd gear - I'm fuzzy on how he did it), when he would climb back on and turn the tractor around for the next row sweep. What he did while I was driving depended on what we were doing - cutting hay or whatever, and whether a truck needed to be alongside for what was coming out of the implement the tractor was pulling.
Got some black and white pictures of the above, and I agree with my wife - I'd never do that with a son, or grandson, of mine - too darn dangerous.....but I sure enjoyed it when I was real young. By the time I was really capable of handling a tractor, it'd gone from fun to a chore.