ko, can be used indifferently in connexion with almost any class of things, animal, vegetable or mineral. But there are other numeratives—at least 20 or 30 in everyday use—which are strictly reserved for limited classes of things with specific attributes.
mei, for instance, is the numerative of circular objects such as coins and rings;
k‘o of small globular objects—pearls, grains of rice, &c.;
k‘ou classifies things which have a mouth—bags, boxes and so forth;
chien is used of all kinds of affairs;