Re: How many decoys do i need?
I was in the same boat. I fully agree that if you hunting fields you can never have to many. But you said you hunt in the estuary so things are different. Take some time to see that the ducks do naturally. Where I hunt I hardly ever see more than a dozen ducks together on the water. Most often it isn't even that many so that is what I try to replicate. You can have great success with a dozen decoys properly set up where the ducks want to be, and good calling.
To answer your question, I would start off with a dozen duck decoys and go from there. Learn to call and proper lay them out with a landing zone in range and your success will go up significantly.
(this applies to ducks on water. Geese and Ducks in fields are a totally different ball game)
I don't shoot innocent animals... Just the ones that look guilty!