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:
"An indefinite pronoun is a pronoun that refers to one or more unspecified beings, objects, or places."
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:
"All failed the test."
"Three failed the test."
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:
"All of them failed the test."
"Three of them failed the test."
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?