Plan B will be equally effective if there is only some semen or if there is a lot, it does not get "washed out" or "overwhelmed." And it will work if the sex is within 72 hours, as well as it can work. But it does have a one in eight chance of failure, so really it is supposed to be used as emergency contraception, not contraception because you haven't gotten onto anything else. If you're having sex five times a day and don't want to get pregnant, it would be like the textbook example of when someone should go on the Pill or get the implant or get an IUD.
Plan B only has an impact on an embryo, not on sperm. It's not a spermicide. You are the one who produces the egg that becomes an embryo, and you will only have one or maybe two eggs in a given month. They can only get fertilized by one sperm, no matter how many oceans of semen they are subjected to. (Besides, by the fifth time your husband had an orgasm, his sperm count in the semen would probably have been zero.) A woman only ovulates one egg at a time (unless her mom's family has fraternal twins, in which case she might have inherited the tendency to ovulate two eggs in a cycle). Where are you in your cycle, are you towards the middle of the month?