well, seeing as how you didn't specify which types of ducks you see in your neck of the woods, it is kind of hard to guess what you may want to have in your spread.
for my area i use 2 dozen mallards mixed with a dozen widgeon. i usually set the widgeon slightly apart from the mallards., usually about 3 or 4 additional yards past my farthest mallard deke and in their own tight little group.
i am planning on adding 6-12 GW Teal to the spread as well.
the variety seems to help, though all the species we shoot here decoy readily into mallard dekes.