Unprotected sex and plan b

So I’m on birth control but me and my boyfriend want him to cum in me, we’re in no position to be having a baby so I’m gonna buy plan b in advance and take it after. My question is say he cums in me multiple times in one day and I take plan b at the end or something. Would that be less effective than if he only came in me once ? Like do you have to take plan b per load haha idk if this is a dumb question ?