If you want to go even deeper into the formation of an embryo, millions of sperm are released each time someone ejaculates. Let's take this number as 400 million (400,000,000).Source The average human male ejaculates 7500 times in his life time.
400,000,000 * 7500 = 3,000,000,000,000 (3 trillion sperm)
Add this variable to your thinking. The odds become even less likely.
Just because the odds are less likely doesn't mean it could never happen. It could happen, because you are here.