We say of something that is not impossible that it is “within the realm of possibility,” or “within the realm of the possible.” The plural form “realms” is so popular in the worlds of fantasy fiction and gaming that it is understandable that many people would refer to “realms of possibility,” but the realm of the possible contains everything that is possible. That’s what its name means. The idea of plural possibilities is already inherent in the word “realm.”

When even serious physicists speculate about multiple “universes” the concept of multiple realms of possibility may sound all right, but it’s neither logical nor traditional.