I think Liam is correct that they will "adopt" other family's ducklings. I have seen a couple of pairs with 20+ ducklings in a few places. Most sources online cite 8-11 as a "usual" clutch size, so I would think 20 would not be from just 1 pair as it seems excessively large. Either way, most often they have far fewer than 16 Ducklings, usually 5-10 in my experience (often quite a few will disappear throughout the course of parenting).