@SmithyDepends on how big the duck. One of the local supermarket duck(lings) will serve two usually...maybe three if you divvy up the breast and legs. I had a couple so undersize that breasts were miniscule and fit for nothing.
On the other hand, one of the big mother breast/leg boxes from costco.com has breasts that are the size (nearly) of my hand and each could serve three generously, maybe four.