How Many decoys to use?

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I've just picked up 2 dozen Avian-x decoys from bass pro in the states and was wondering how many decoys everyone uses in the marsh? I have 5-6 dozen decoys now and am not sure how many to use. Would it be better to use them all or just put out 2 or 3 dozen? also one of my hunting partners just got another robo duck. is two better than one?

Thanks.