Question

even though each pollen grains has 2 male gametes why at least 20 pollen grains are required to fertilize 20 ovule in a partial 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. For one Zygote to form, One male and one egg is required

Answer 2

During double fertilization, one of the male gamete of pollen grain fuses with egg cell, while other male gamete fuses with secondary nucleus. Thus to fertilize 20 ovules in a particular carpel, 20 pollen grains are required.