I don't know how much you should use, but I do know this:
Cut your lemon grass stalks into BIG pieces. Big enough that you can easily find them before serving, so you can fish them out. They are not edible. It's like trying to eat a tree branch.
Just re-read the suggestion about mincing the lemongrass. This might work, if it's minced into microscopic pieces. I cut it into 1" pieces one time, and the dish was completely ruined. Other recipes I've seen suggest 4" - 5" pieces, used just for infusing flavor, and then removing them.