Question

even though each pollen grain has 2 Male gametes,Why atleast 20 pollen grain are required to fertilize 20 ovules in a particular carpel?​

Answer 1

Even though 2 male gametes are there in one pollen grain, 10 pollen grains are required to fertilise 10 ovules. This is due to the presence of double fertilization in angiosperms. ... The other male gamete fuses with the polar nuclei and forms the primary endosperm nucleus

Answer 2

From the two male gametes in a pollen grain, one fuses with polar nuclei to form endosperm (Triple fusion)  and the other fuses with the egg cell to form the zygote (Syngamy). Since only one pollen grain is used to fertilize the egg cell, if there are 20 egg cells, 20 male gametes are required.