The ones that come as tree drops are only single feathers as well. At least the bird kill drops are 3. The black forest crypts can sometimes give 10 or more in a single chest... not that often, would be nice if they gave more often.

That said I usually have a chest full of feather by the time I start making frost arrows. But I avoid using them early on... only for boss fights and not that many. Rather save them for better arrows. If they dropped more often I would probably start making fire/flint arrows right away ha ha.