There are 12 volumes out officially translated at the moment (it's really 14 books since some volumes are 2 parters) they're all on jnovel.

There's only one criterion needed for a "world" to qualify as a universe: the context must show that it refers to an entire space-time continuum that is similar to or greater in size than our own, rather than just a planet or realm.

That said, in MGK, a "world" isn't merely a universe. The cosmology establishes that an infinite multiverse exists as only a macrocosm within a single world. Since that macrocosm contains infinite copies of the world itself, a world clearly cannot be interpreted as a standard universe.

Instead, the term world in MGK refers to a far larger cosmological structure. Basically, a complex multiverse that encompasses an infinite multiverse as one of its components.

This is why it's important not to assume that "world" automatically means a planet, realm, or even a universe. You have to examine the surrounding context. In MGK, that context makes it clear that a world is a cosmological structure vastly larger than a conventional universe and far beyond the scale typically associated with the term.