Every English dictionary in the world categorizes numerous as an adjective. And quite rightly: it mostly is. But a recent development has seen it pick up a second life as a determinative: a word like all, many, most, none, several, some, that, and this. Crucially, (i) at least some determinatives can form a noun phrase all on their own, as in All were approved, and (ii) at least some determinatives can make up a full noun phrase when accompanied by a partitive of phrase (but no head noun), as in some of my best friends. Adjectives cannot perform either of these feats: *Good were approved and *Happy of my friends liked it are wildly ungrammatical. Articles can't either: the articles a(n) and the are special determinatives that have neither of the properties (we don't find *The was very thoughtful or *An of my friends did it). But in recent writing, numerous is turning up (albeit rarely) with both properties, and thus taking on the syntax of a word like several.
Yesterday Rodney Huddleston in Australia captured a case of the first sort, where numerous on its own acts as a noun phrase (I underline the clause that has numerous as its subject):
Many countries treat illegal immigrants and asylum-seekers far more harshly than Australia, and numerous impose mandatory detention, a provision explicitly provided for in the refugee convention. [The Australian, 23 August 2011]
And thereafter Brett Reynolds in Canada rapidly found two cases of the second sort, with a partitive of-phrase (I underline the noun phrases):
Numerous of the former do advocate a discipline, the cultivation of a form of ability… [2000, academic English]
As numerous of the more thoughtful teens interviewed observed, a lot of L.A. slang comes from… [1996, news reporting]
I agree with Brett, who documents the findings on his excellent blog English, Jack, that the Australian example feels unnatural, close to ungrammatical. In other words, this is a new phenomenon, and Brett and I speak a variety or stage of English in which the change has not yet occurred. The modification is slight, and quite natural: we are easily able to see what the Australian example means. But we wouldn't ourselves write it, at least not yet.
You will find something similar is happening with various, and other words too [update: the philosopher James Dreier just mailed me some examples of the same thing with multiple].
This isn't just a lexicographical triviality like the loss of the word charabanc or the arrival of the word permalink. There is an interesting theoretical observation to be made. Many grammarians say that noun and verb are "open classes" to which new members can be added, but small classes of words like the determinatives are closed classes. But they are not closed. They are taking in new members. Just not very many, and not very often.