"Andrew, a question. At what point does an ensemble become a chorus? Così had the smallest chorus I've ever seen in an opera. I didn't count them, but there couldn't have been more than 10 or 12."
In this opera, the chorus is usually (but not always) offstage. All it does is to sing "Bella vita militar!" twice in Act 1 scene 2, and then only once in the dénouement near the end of the opera. I haven't seen this production, so can't really comment on the number of singers, but this really is an opera in which the chorus has very little to do.