How many sperm cells are needed to fertilize an egg?
It takes just one sperm to fertilize an egg and achieve a pregnancy. The odds are always stacked against any single sperm cell making a grueling journey to the egg to fertilize it. Nonetheless, to offset the normal situation, a large number of sperm cells are produced when needed.
An immediate chemical reaction follows the penetration of one sperm cell into the egg. It protects the egg from the penetration of other sperm cells.