Most I got by trying for 3 mins was 49, but that's probably not the max. Theoretical max would be 64, if every piece covered every other.
A technical question: Do the two bishops have to be of different colors?
EDIT: Now 50.
No. For the game to be easily generalized and 25 knights or whatever be possible to put, there is no color limitation.
What do these things have in common? Nerves, emotions, morality, prices.