GHG, Final Approach, Game winner...best to worst in that order. I own some of each. I usually only use 12-15 dekes, never more than 3 dozen. Sometimes we bring out a mojo too. I hunt mainly flooded rice stubble fields. On the other hand, my uncle hunts a large lake and never uses less than 100 dekes. Number depends on the area. I have mainly mallards, teals, and a few woodies and pintail. I'd say start with 2-3 dozen and see what you need to fix.
Hope this helps some.