Question

why do fish and frog lay egg in hundreds , whereas a hen lay only one egg at a time

Answer 1

They lay thousands of eggs so they can ensure at least few of them to be living because most of them get destroyed due to unfavorable conditions.
Frogs lay in water and there is a chance many of them get eaten by fishes or snakes.
Hen lay only one because they lay on land and there is less chance for it to get destroyed.

Answer 2

because of 2 reasons:
1.Frogs eggs r very small.
2.they lay i them in water and most chances are there that the egg will be carried by the water hence decreasing the probability of fertilization