Question

why do fish n frogs lay eggs in hundreds whereas a hen lays only one egg at a time...??

Answer 1

Actually female fishes and frogs lay eggs in hundreds and also release sperms in hundreds to fertilize the egg
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This is because there is a chance of egg being eaten by predators and even getting destroyed as they don't have protecting layer.So, laying eggs in hundreds will give a chance for few of them to survive and grow into adult.
But egg of hen has a protective layer hence, it lays only 1 egg

Answer 2

All animals want to make sure that at least some of their young ones reach adulthood and reproduce themselves.
some animals do this by looking after a few young ones until they can fend for themselves.

some animals like fishes and frogs produce hundreds of eggs which they leave to develop by themselves. Most of them dies, but a few will survive.

hope it helps.