How many fingers on the brake lever? I have seen people stop with one finger, two fingers or the whole hands on the levers. Is there a recommended number of fingers on the brake levers when stopping/anticipating an emergency stop? Do brake types or riding style (DH, road ..) affect this number?
For downhill and all forms of trail riding (freeride, enduro, all mountain etc) the prefered way is 1 finger (index) on each lever. Anything else can be dangerous. If the brakes are not powerful enough for dealing with this then they either need fix or replacement to proper brakes for this type of riding.

Also, riders position the levers closer to the stem (leaving 1 to 2 inches space between the grip and lever clamp) in order to enforce that only 1 finger can go on the lever. This also helps the finger pull the lever from its most powerful spot.