Question

Why do fish and frog lay more Number of eggs where as cow and human being usually give birth to Only one at a Time

Answer 1

because for fishes the participation of egg and sperm are more where as in human only a egg can be involved so its like that

Answer 2

the animals like fish and amphibians fertilization takes place at outside of body . this type of fertilization is called external fertilization.Hence it releases many sperms.
whereas in the animals fertilization takes place at inside.
so human and mammals give birth to only one at a time
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.i hope u understand