1. I think racers on tracks use only one finger because their front brakes are so powerful and they don't need to stop - no personal experience.
2. Off road riders use differing technics and bikes. Since I don't do that, have again no experience or advice.
For road riders, had this discussion some time ago about number of fingers on the front brake lever on another forum. (MSF directive was to use all four fingers always.)
What I took away from that thread was 'use the least fingers you need to get maximum braking from your bike.:" For me, on my bike, that is the first two fingers. Confess they are not always over the lever - but they migrate there with no thought at the slightest perceived threat or possible threat. I can roll off throttle and add brake at the same time, or vice versa, which also contributes to smooth riding. I also like having the progressive brake control rather than a stab at the brakes from my hand off the brake.
Another poster suggested twice his hand reaching the front brake lever was faster than his brain. Can we all agree that the brain has to perceive a threat BEFORE the body can take any action? Can we also agree a four finger stab on the brake lever might not end well?