Its not practical to carry a "spread" around with you while out hunting. One decoy is light and portable and adds confidence for a bird coming in looking for that hen he heard. If you were in a stationary spot, in a blind or something, more decoys might be reasonable. The most I have used at once was 3 on 2 different occasions. 1 Jake and 2 hens, both were successful hunts but I don't think it was just the decoys that did it. One of those hunts was in a field and another guy came in and set up a blind and 9 decoys across the way from me. The birds elected my 3 decoys over his 9, why, I don't know. I have called in plenty without any decoys too. Ducks are flying high overhead and can survey lots of water so its a bonus to have a visible spread, a tom only needs one hen to make him happy for a while.