It depends on whether it makes sense as a single, unified question.
The deciding factor here can't be a number -- "It's OK to ask about the inspiration of five characters, but six is too many." That would be arbitrary.
This rests upon a simple (but subjective) test: is this one single question, that makes sense and will have a single cohesive answer? Or is it lumping many questions together, when they would make more sense on their own?
A hypothetical question "What inspirations are known for characters in Alice in Wonderland" would definitely be too broad; it would be asking for a list of character-inspiration explanations, that have no real relation to each other. Whereas "What characters in Alice in Wonderland are based upon real people" seems like a narrow list, and one based on a very significant common element -- no different from, say, "What mathematical paradoxes are discussed in Alice in Wonderland".
In general, I believe we need to be wary of arbitrary questions - questions that arbitrarily assume that a certain significance, inspiration, meaning, might possibly exist in a piece, and ask that the community prove or disprove this conjecture. It's kind of fishing for a question -- "I don't know if there's anything interesting here, but I'll ask, in case it turns out there is."
Questions like this are extremely easy to ask, are often unanswerable, and produce site content that feels like a list of undercooked stabs in the literary dark.
Broad questions, e.g. requesting inspiration sources for a large number of characters without clear focus and justification, are very likely to be arbitrary fishing questions. I would not want to see future blanket questions justified by "But on Meta we said it's OK to ask about a large number of characters."
On the other hand, some questions quite rightly cast a wide net. Sometimes you're asking about something the repeats broadly, that has many instances, and the entire point is to ask about all of them. This will usually be something very specific, and that's why asking "Tell me about this specific thing across a broad range" can make a lot of sense, and be well scoped.
So, I would say the guideline for whether a question on character inspirations would be something like this:
- The OP should explain why it is reasonable to expect to find the kind of character inspiration s/he's looking for.
- The OP should be asking for something that makes sense as a single, cohesive answer, rather than a list of unrelated answers, each pertaining to a different character.
If a question satisfies those, then it doesn't matter how many characters the question covers. If it doesn't, it's likely to be a poor question, or too broad, or both.