No, combien d’eux cannot be used in place of combien d’entre-eux, just like combien de nous cannot be used for combien d’entre nous.

At best combien d’eux might translate “how many from them” like for example in Tu en veux combien de moi et combien d’eux, i.e. “how much of it do you want from me and how much from them” but outside such a context, that form would likely be avoided because it is homophone with combien d’œufs, “how many eggs”…