First, why is it even a problem? As time progresses, more universes are created in an exponential fashion. However, no matter how long you wait, the number of universes will always be finite. Why not just assign probabilities taking all universes into account, if there are only a finite number of them
In a single universe, cows born with two heads are rarer than cows born with one head. [But in an infinitely branching multiverse] there are an infinite number of one-headed cows and an infinite number of two-headed cows. What happens to the ratio?
Except, there are never going to be an infinite number of either type of cows; the number will always be finite, and I'd expect that the one-headed cows are simply more numerous than the two-headed cows.
Second, why do physicists not simply accept that probability has no meaning in the multiverse? Probability, for sure, is very useful in physics, but why feel the need to accept it as an immutable and proven law? For a long time, philosophers have questioned how exactly probability should be taken, and whether or not it is a valid tool to use. For example, ideas like the problem of induction (potentially) show us that the scientific method is not completely justified - we just use it in a pragmatic way. Why not accept the same with probability?