NFHS Rule 7-1-4 Illegal Holds/Maneuvers
"Pulling back as opposed to grasping the thumb or 1,2, or 3 fingers of an opponents hand"
In other words it is perfectly legal to hold 1 thumb, 1,2 or 3 fingers. What you cannot do is pull them back or exert pressure against the joint of fingers. A couple of years ago it used to be illegal by application to simply grasp any less than all 4 fingers. By rule today it is no longer an automatic infraction if you grab less than 4 fingers.
As far as what is considered "pulling back" that is a judgement call on the part of the official. Another thing to consider is that the wrestler whose fingers are being grasped could be moving their arm in such a manner to cause their own fingers to be bent into a dangerous position. In this case the rule book clearly says that a wrestler shall not be penalized when the opponent turns a legal hold into an illegal hold.
Most times it is just easier for the official to try to clear the situation verbally, or call a potentially dangerous stoppage when a situation as described above occurs when a legal move is turned illegal by the opponent.
So again, you DO NOT have to have a certain number of fingers grasped, it is only illegal if the finger(s) have been pulled back in the judgement of the official. Understand that this rule and several other just like are there to prevent injury and in some cases to prevent an unfair advantage. The rule is not there so kids can score cheap points.
