There isn't a good answer to your question. The number of decoys and the pattern they should be placed varies from place to place. You might want to invest in a dozen which is a good start and maybe the minimum you could get away with on some places and more than enough for others.
Watch real birds when ever you can , in what formation do they land and imediately after. What do they look like at reast, feeding etc. There are no shortcuts (mmmm maybe that robo decoy) in becoming a good waterfowler.