As many utilitarians have pointed out, the act utilitarian claim that you should ‘act such that you maximize the aggregate wellbeing’ is best thought of as a criterion of rightness and not as a decision procedure. In fact, trying to use this criterion as a decision procedure will often fail to maximize the aggregate wellbeing. In such cases, utilitarianism will actually say that agents are forbidden to use the utilitarian criterion when they make decisions.
I like this post. Another post I’d recommend on related topics is Act utilitarianism: criterion of rightness vs. decision procedure. Unfortunately it lacks a summary (though it is at least quite short), but here’s one excerpt:
I also like and recommend Purchase Fuzzies and Utilons Separately.
Also see https://​​www.utilitarianism.net/​​types-of-utilitarianism#multi-level-utilitarianism-versus-single-level-utilitarianism
For more on criterion of rightness vs decision procedure