Question

why do frogs and fishes lay hundred of eggs at a time??

Answer 1

Fishes and Frogs lay hundreds of eggs because by the time they hatch the chances of the egg getting affected by various predators and survival of the young ones reduce.
Hence, its mainly to increase the chances of survival of their babies.

Answer 2

because the egg are so small that it is attacked by many animals larger than it.they lay 100 of eggs so that at least some can be born.