you're spread has a lot to do with what and where you're hunting.
if I'm hunting locally I hardly ever use more than 2 to 2.5 dozen mallard decoys. we simply don't get the numbers
now diver hunting the lakes. no spread is too big
I just bought 3.5 dozen more divers yesterday.
and don't be afraid to use geese decoys when duck hunting
I've killed an untold number of mallards over goose fakes