I think you've read one too many Billy Mumphrey stories.
The way I see it making sense is if one means alone, single-handedly.
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Sign up to join this communityThe expression is a mildly critical way of saying you have read Billy Mumphrey stories to excess.
Suppose the perfect number of Billy Mumphrey stories to read is seven.
If you read only six, you have read too few—one too few.
If you read eight, however, you have read too many—one too many.
We use the one too many to take the sting out of the criticism: it suggests you have only overindulged to a slight extent. It's probably used most often with respect to alcohol consumption.
Bob, you're getting just a little loud. I think you've had one too many.
If one person says to another, "You've watched one too many horror movies", that would imply that the second person displays some kind of paranoia or something else that may (or may not) stem from too many horror movies. It may be nothing more than the second person is a bit "weirded out" because her phone is dead. In that case, the dialog really has nothing to do with horror movies except that dead phone lines play a part in a lot of horror movies.
It's not always literal.
One too many drinks? Yeah, that's actually referring to alcohol. One person is telling another that he's drunk.
It's also common to hear, "You've been hit in the head one too many times". No one is actually going to use that line with a head injury patient.
"I won't loan you money, I have been stiffed one too many times." Eh, he's recognizing a pattern, ONE has nothing to do with it.
Guy stands up to leave a great party. Host says, "Ha ha! You've been divorced one too many times, my friend!" Maybe he's agreeing that his friend should leave, maybe he's chastising him for being under the thumb of his wife.
This is a hard one to articulate, "one too many" can mean a lot of things. For this one I can only recommend that you look for context and make no assumptions.
EDIT: Todd mentions "tipping point". That works if the speaker isn't trying to crack a joke.
Further EDIT: So your favorite jeans are a bit too snug? "Ha! One too many Margaritas!" Again, not literal although not terribly far off the mark either.
"One too many" is an idiom meaning that a tipping point has been reached.
I don't know the complete context from Seinfeld, but what is being said that Elaine's familiarity with Billy Mumphrey is affecting her perspective.