What is the chance of getting pregnant when a lot of sperm gets inside?

Dificult to say: It depends on the condition of the sperm, and the condition of the female pelvic structures from the uterus to the fallopian tubes.

That is the way : Fertilization and pregnancy are result of sperm entering vagina, swimming up into the uterus (womb) thru the cervix then into fallopian tubes where they can fetilize the an ovum (egg). That is why a condom should be worn unless you want to get pregnant.