Should the concept of a quantity of objects be considered singular or plural?
My gut says that
"Fifteen options is too many"
is more correct than
"Fifteen options are too many"
but I can't articulate why the subject feels like a singular concept.
Which is correct?
In brief, this is an example of notional agreement. If you're thinking of the options as a unit or quote, say
Fifteen options is too many.
If you're thinking of the options as individual, countable options, say
Fifteen options are too many.
First, let's lay out the rule you already know: usually you want your subject and your verb to agree in number. According to this paradigm, you would say:
Fifteen options are too many.
Six options are too many.
?A range of options is too many.
The last example shows something interesting. According to the previous rule, "a range" is singular and therefore the singular verb "is" should be used. However, "a range of options" feels like a plural concept - there have to be multiple options to have a range. Subject-verb agreement on the basis of semantics or feelings - like the one you have in your question - is called notional agreement.
Merriam-Webster gives several examples where plural unit nouns with numbers might take a singular verb.
Ten dollars is the cost of admission.
Is five miles too far to walk?
Two plus three makes five.
The explanations for precisely when to use singular or plural verbs vary. Some prescriptive grammarians ignore notional agreement and treat violations of conventional S-V agreement as an error. Some, like editor Erin Brenner, create very specific rules for when to apply notional agreement:
Specific amounts of money take a singular verb, while vague amounts take a plural verb.
Whatever the exact explanation (here's one more), the logic here is that you use singular when there's a semantic reason for thinking of the noun phrase as singular and plural when there's a semantic reason to think of it as plural. My own inclination: the Merriam-Webster examples and your example ("Fifteen options is too many") treat the amount (ten dollars, five miles, two plus three, fifteen options) as notionally singular because they feel like a unit quantity. Conceptually, someone asking after the quoted amount would likely treat the figure as singular:
How much is the cost of admission?
How many options is too many?
When thinking in terms of a unit, figure, or quote, singular works.
A small shift in perspective will mean that we are thinking of the plural collection of items, rather than a unit quantity:
How many dollar bills are required to pay admission?
What options are the most redundant to you?
Both questions require thinking about multiple individual items rather than a set number. Hence, with quantities, notional agreement should be considered alongside conventional subject-verb agreement.
I see what you mean. Depends how you look at it.
The number 15 for options is too many/big. We should have a smaller number of options.
Fifteen options?! That (number of options) is too many.
Or
These 15 options are too many/numerous. We should have fewer than 10 options.
Bottom line: If it is a plural subject, it should conjugated as such. As to why you feel like numerous entities should 'count' as a singular concept, I suggest you explore concepts like mass nouns or noun phrases. Note that those concepts may not directly apply to the example you provided, but are I would argue contextual neighbors.