As "none" and "some" are pronouns, so can specific numbers function as pronouns:
How many students failed?
In "none failed," none is a pronoun.
In "seven failed," seven is a pronoun.
But in what category of pronoun do numbers belong? The categories I know of are personal, demonstrative, interrogative, indefinite, possessive, reciprocal, relative, reflexive, and intensive—and specific numbers would not seem to belong to any of these.
Best Answer
In this case, I believe that numbers, when used as a pronoun (because English is just so flexible), would be an indefinite pronoun, just like "some."
According to Wikipedia, most frankly:
Which, essentially does work with numbers, doesn't it? Considering how "numerical pronouns" are used, they function in a sentence exactly as any other indefinite pronoun can - in fact, you might just be able to substitute a number for any indefinite pronoun in a sentence. For example:
Also, if you were to add something to the sentence to make it more obvious that the numbers are actually used as pronouns, not adjectives, you can get:
Then, it becomes quite obvious that here, the number would actually be a pronoun, wouldn't it?
P.S.: If all else fails to convince you, remember that "one" is an obvious example of an indefinite pronoun - after all, if "one" works, why wouldn't "two", "three", or any other number work?