If you’re going to talk about other universes, dictionaries gonna have to change their definition of the word “universe”, which is now defined as, everything, known and unknown.
Only one “everything”.
"What we have thought to be everything, may actually be a tiny part of a much grander cosmos. And that grander cosmos can contain other realms that need to rightly be called universe just as our realm has been called universe. Which means you have many universes, multiple universes, which we call the multiverse." - Brian GreeneThat's how BG tries to explain it away.