The longer you wait the easier it is for the vet to evaluate their sizes, which is also a very important thing that can come from an x-ray. It isn't as exciting as knowing how many but it is important. I would go around 55 days or after. It's tough to wait, I know. Either way it will be super exciting to see them, even if it is a few days after what you'de like.
(either way the waiting for pups will drive you so darn crazy, it goes
so slow at the end of pregnancies.)