In Classical Greek, there was singular, dual, and plural. The Dual dropped out by the Hellenistic Period (though it appears occasionally – I think Josephus uses it once or twice).
Anyway, I moved on from Robertson to Moulton’s discussion of nominal syntax and his comments about the dual caught my eye. They are comments that would make a field linguist irritated:
On the subject of Number there is one obvious thing to say—the dual has gone. Many Greek dialects, Ionic conspicuously, had discarded this hoary luxury long before the Common Greek was born; and no theory of the relation of the Κοινή to the dialects would allow Attic to force on the resultant speech a set of forms so useless as these. The dual may well have arisen in prehistoric days when men could not count beyond two; and it is evidently suffering from senile decay in the very earliest monuments we possess of Indo-Germanic language.