In a flower ovary consists of ovules.Ovule has an egg cell.When pollination occur among several pollen grains only one pollengrain enter the ovule and fertilize the egg.
After pollination ovary is converted to fruit and ovules are converted to seeds. Only one pollengrain has fertilized the egg cell in the ovule,so only one seed should be formed.But how large number of seeds formed,from single pollengrain?
I guess fertilization can occur many times within one plant. Some flowers might have several ovules. Each ovule contains one ovum. When pollen grains land on the stigma, more than one pollen tubes can be formed and each carries a male gamete to one ovule.