I'll take a stab at it.
I was raised kinda rough and my grandmother and parents made us work. I'd help Mawmaw pluck wild ducks in the shed. I now pluck my own domestic ducks and sometimes wild ducks.
As far as domestic goes....I'd guess that it would take from 15 to 20 nice size ducks to fill an old pillow case (which is what we have always done). When Mawmaw had enough to stuff a pillow, she would use tough denim fabric to make one, so that is what I do.
I'd guess that when you have enough feathers to stuff 2 pillows, you could probably estimate that you could bat a quilt....I mean, it will be stuffed much lighter and then quilted in place. You will need a good amount, but maybe you could just start saving them and put the word out to a friend that also slaughters water fowl.
I can tell you that when my godfather went hunting, we'd have to pluck many, many ducks before we had something worth talking about. Much smaller birds.