That is, if a quantum world is something whose existence is fuzzy and which doesn't even have a definite multiplicity - that is, we can't even say if there's one, two, or many of them - if those are the properties of a quantum world, then is it possible for the real world to be one of those? It's the failure to ask that last question, and really think about it, which must be the oversight allowing the nonsense-doctrine of "no definite number of worlds" to gain a foothold in the minds of otherwise rational people.

I'm downvoting you for the tone and the constant application of insults.

And also for confused thinking. MWI advocates don't believe that we collectively inhabit a "single" world. The first picosecond after any single quantum event I cause, surely people on the other side of the world haven't had the time to branch yet in response to that quantum event -- the light-speed barrier alone prevents them from having so branched.

So at any given time, some people have branched, and the rest of the planet hasn't yet branched alongside them. Some of those local branches will even merge back into one, and some will not. So that alone shows there's no "definite", as in universal, number of branches, same way there's no definite, as in universal frame of reference for movement or acceleration.

Your argument is effectively the analogous of saying "if the Einsteinian relativistic school of thought says there's no definite frame of reference, only an infinite frames of reference with no definite reality in them, how is it possible for our own frame of reference to be real? You relativistists must have merely not thought this through, that's why you believe in such obvious nonsense"