How many frags would it take to destroy a tank?

Only under the most bizarre circumstances would you successfully destroy a tank with frag grenades. Frags throw off tiny bits of metal, known as shrapnel, and tanks are more than sufficiently armoured to withstand it. When tanks had been defeated by soldiers with frag grenades, it's because the soldiers were able to climb onto the vehicle, force open the hatches, and drop the frags inside the vehicle, killing the crew. Unless some freak accident caused the ammo carried inside the tank to go off through sympathetic detonation, however, the tank itself remained fully serviceable.