Oh my freaking gosh! It's a plot!
And thank you,
@Mathiason, for another plot point!
In reality though:
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[*]Mature egg comes out once in a month (menstrual cycle).
[*]This egg lives for around 12-24 hours at most. It can only be fertilized during this short period.
[*]For a fast-moving sperm cell it takes at least 30 minutes for sperm to reach an egg, i.e.
you need at least 30 minutes to become pregnant after you had sex. But that's the best possible scenario. Normally fertilization happens around 18-24 hours after. And don't forget: you need to hit this 12-24 hours window when mature egg comes out.
[*]Among 40-150 million sperm cells released during the ejaculation
only few hundred ever come near the an egg (and have a chance to fertilize it) due to many natural barriers in woman's body.
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But this is a fantasy manga. Probably all those girls have their eggs "at the ready", zero barriers (both mental and physical) and an MC has a never-ending stamina and increased fluid secretion (95-97% of seminal fluids released by male do not contain sperm cells, it's just a "seminal plasma", a solution that keeps sperm cells viable).