We were informed of this when the original vaccines came out.
When was that? In June 2021 the CDC hadn't yet recommended any boosters for anyone, and only allowed that people "most at risk
may need one".
https://archive.ph/SDHy2If you had gotten vaccinated, you would have gotten a vaccine card that had 5 spaces on it for when you got shots implying that there could be more after the initial 2.