How Do You Define Probability if Everything That Can Happen Is Going to Happen?

In many worlds, everything happens. And so i can no longer say that the probability of this happening is forty % or 60% because they both happen in some universe. How do you define probability if everything that can happen is going to happen? I think this is the right question. But then let's talk about how to use many worlds interpretation, e know, as a functioning theory of quanty mechanics. Because something that is a bit slippery for me is that, in the copethagan interpretation, i know what a probability means.