Even just a little bit of sperm can get a woman pregnant if it gets into the woman's vagina.
You can also become pregnant even if your boyfriend or husband ejaculated onto their fingers and then used the finger to finger you through your vagina.
So it's possible to become pregnant without the guy even inserting his penis into the woman's vagina.
It only takes one sperm to fertilize the egg in a woman and you need very little sperm to become pregnant.