The Cambridge Grammar of the English Language (Page 385) has this to say about "cardinal numerals":
7.6 Cardinal numerals
The cardinal numerals are primarily determinatives but they have a secondary use in which they inflect for number and hence belong in the noun category: They set off in threes/enrolled in their hundreds. In practice, only low or round numerals are used in this way. 24
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24Numerals are often used metalinguistically, as the names of symbols: They added a ‘3’ before all the Brisbane telephone numbers.
What's the part of speech of the following numbers in CGEL?
Now, these orbital images tell me that the hostiles' numbers have gone from a few hundred to well over 2,000 in one day. <Avatar (2009)>
Since they don't inflect for number, they don't seem to belong in the noun category. Nor do they look like determinatives. The only choice left is to say that they are being used metalinguistically, but this doesn't seem right either.