In real life tanks of course carry a limited supply of shells, and have to be very careful what they use them for. Not like our tank drivers who start shooting happily into every hillside from the moment they appear.
Not only did normal WW2 tanks carry a restricted number of shells, but some of them would be anti-personnel, others would be anti-tank, and some would be incendiary, so the actual supply for one particular kind of job would be even more limited.
I think cutting down the ammo supply to 15 or even to 12, would cause very little problem for the tank drivers, and it would prevent them from being too destructive.
Alternatively one could ban tanks altogether from say dogfight and bomb-carrier games, where they don't play any important role anyway.
But judging from the immense popularity the tanks seem to enjoy, I guess I would be the only one in favour of that.