Why do fish and frogs lay eggs in hundreds whereas a hen lays only one egg at a time?

Though, fish and frogs lay hundreds of eggs, all of the eggs do not get fertilized. Hence, very few new frogs are produced.

All eggs do not get fertilised because

• Frogs lay sperms in water. Many of the sperms get washed away due to water movement. Thus, egg is not fertilized
• Many eggs are eaten by the organisms living in water
• Even after eggs are fertilized to form larva, they are eaten by other organisms.. Thus they do not form new frogs

Whereas, in case of a hen, internal fertilization takes place and hence the survival of the baby which would hatch from the egg has much higher rates of survival as compared to those in case of a frog. Hence, hen produces only one egg.