There is no way to know this unless you know how much they will waste as chicks (and I guess as adults if you don't have a good feeder). If you want to take the easy way, just calculate 1/4 pound of feed per day per chicken. That will work pretty well for when they are full grown while also factoring in a good amount of waste when they are younger.
You just curious, or are you trying to buy all your food upfront?