Answer: there is no upper limit.
Synopsis: on The View, a horrifying chit chat show where Barbara Walters stands out as an intellectual giant, their new co-host -- and I can barely type this, it makes my brain hurt so much -- says she doesn't know if the Earth is flat or not.
You may wish to reread that. I don't recommend it, though, if you don't want your cerebrum to explode outwards in all directions at the speed of light.
The link above has a video on it, in case you simply can't imagine that anyone who would land a job as co-host of a national TV program could actually not know for sure what the general shape of the planet is. Sherri Shepherd, the person in question, knocks it out of the park, though. This, after saying she doesn't "believe in evolution, period."
Hey, that's something we share! I don't "believe" in it either. I don't believe in anything, especially rock solid facts. I know evolution is real. Belief is for things without evidence.
Anyway, when Whoopi Goldberg (who is actually pretty smart) presses her on this, Ms. Shepherd demurs, saying that it's more important for her to know how to care for her son. This is almost legitimate. Almost. But it misses. If this were a thousand years ago, and she were toiling in a cave someplace with no access to information and spending 20 hours a day trying to keep her family fed, then sure, some knowledge may simply be too esoteric to be useful and, worse, distract from the actual task of survival.
But that isn't the case. Here we have an actress and singer who is living, if I read my calendar and atlas correctly, in the 21st Century in the United States. Has she never seen a picture of the Earth from space? As it happens, a vast majority of people in the U.S. can hold a job, care for their family, and also know that the Earth is, y'know, round. Some people (though sadly, not enough) also know it takes the Earth a year to go around the Sun, that gravity makes things fall, and that DNA is a big molecule in which genetic information is coded. None of this is needed to feed your family (unless you are a science writer), yet humans are in general capable of handling a vast amount of information not directly pertaining to immediate survival.
For someone in Ms. Shepherd's position to not know the Earth is a ball is really just beyond my ken (sadly, not believing in evolution is par for this country's course). *
Now, there are lots and lots of people out there who lack an education (and there is blame to go around aplenty, there) or for whatever reason may not understand the Earth is a sphere. But then, how many of them get to be co-host on a TV show seen by millions of people every day?
Look, don't get me wrong: people have priorities, and for some, I understand that even basic knowledge of the Universe around them isn't among those priorities. But a co-host on a TV show like this should be for role models, for people who will have an informed opinion and who will make the viewers think. Yes, there are demogogue shows with bloviating ignoramuses as hosts, too many to name. But The View is not really one of them. Shallow, yes, but not -- pardon the expression -- geometrically flat.
The producers of The View need to seriously rethink this. But then, it's not like they've shown brilliance in discretion when they've picked past hosts. Maybe they just want to drum up controversy. In the sad world of network TV, that would also be par for the course.
But this... sigh. It goes to show what a long, long road we have ahead of us.
Tip o' the Mercator projection to Mike's Weekly Skeptical Rant, which is really ad hom, and has much NSFW language.
* I will give her some credit, though: when asked what she would do if her young son asked her if the Earth were flat, her reply was, "I'd look it up"." That is the right answer. However, given her comment on evolution, I fear for where she would look it up.