In Paul Benacerraf’s delightful “What Numbers Could Not Be,” he says:
(1) There are seventeen lions in the zoo
is not to predicate seventeen-hood of each individual lion. I suppose that it is also true that if there are seventeen lions in the zoo and also seventeen tigers in the zoo, the classes of lions-in-the-zoo and tigers-in-the-zoo are in a class together, though we shall return to that. It does not follow from this that (1) predicates seventeen-hood of one of those classes. First of all, the grammatical evidence for this is scanty indeed. The best one can conjure up by way of an example of the occurrence of a number word in predicative position is a rather artificial one like
(2) The lions in the zoo are seventeen.
But in a number of European languages, that construction is normal. For all I know, you can’t say it about lions, but “we are nine,” stated to a hostess at a restaurant, is normal.
Let’s hope our discoveries about mathematical ontology don’t depend on the fact that we’re speaking English.