Morning after pill??

This might seem like a stupid question, but I’m just confused 😂 so last year in health class we were talking about different contraceptive methods, and I’m pretty sure my teacher said that the morning after/plan B pill or whatever only works the first couple times you take it, and then it doesn’t really work as well? However, I was talking about it with my mom a couple weeks ago, and she said that’s completely wrong and you can take it as many times as you need and it should still work. So what’s the truth?? Have I been screwed over by the public education system once again??