The model is not dependent on there being infinite beings. Admittedly, if you believe that there are the same number of people in two universes if there is a bijection between people, then you'll get the result that this doesn't work when there are infinite people. This is the way infinities are usually dealt with in mathematics, but I would argue that this is flawed, that it is actually possible to have one infinite that is twice another.

Anyway, there is no requirement to know exactly how many observers exist. We can just guess.