Benefits asymptote, so you'll stop at the point where the marginal benefit of an additional shot is less than the marginal financial cost, marginal opportunity cost and marginal cost of side effects all combined.

That's definitely not the case before two and may not be the case before three based on data we've seen. It might be the case before four.

And this doesn't factor in declining immunity over time. It might make sense to keep taking shots every half a year forever, until COVID mutates into something less deadly perhaps.