Is the believer's degree in Paradise for him alone to inhabit, or do other believers who share the same degree partake in it with him?

A single level in Paradise may encompass a large number of believers, as indicated by the elevated status of the Mujahideen in the hadiths. Their sharing of a level does not imply constriction; rather, each will have a vast, independent kingdom, so spacious that even if all beings were to gather in one of them, it would accommodate them.