I've done very well with a smaller spread as you describe(under 10 dozen),no real magic number of decoys that will work in any situation.
Some days a realistic small spread will do better than a large mixed or what is referred to as a junkyard spread and vice versa.
If you hunt traffic or the X will also make a huge difference as well.
"No man's Second Amendment rights are safe when the legislature is in session,especially when Democrats are in charge" Mark Twain
I carry a gun,because a cop weighs to much