only 20% chance each cycle

Just did my first iui on 3/27 my doctor told me that we have the same percentage of conceiving as a normal couple which is 20%. This got me thinking, how do so many people have children if the percentage is so low each cycle? I also watched a documentary called the great sperm race, about how one sperm must fight all the odds and how our bodies actually fight off and try to kill every last sperm. I'm feeling so discouraged, how is pregnancy even possible?!?!