You should not limit a wizard can be used. If you
lose the game because of a wizard on the 20th
question, it is either because :
a) It was a buzzer
race
b) You already used your wizard
For part a,
buzzer races is part of the game, it is something we all
have to deal with.
I think for the most part
though, part b will be the reason why you lost on a
wizard play. A team should not be pentalized because
they saved their wizard "play" while the other team
used it earlier.
DJB