Feed them and watch. If they demolish all the food within a few minutes then it is too little. Different breeds eat different quantities. Last year I had 24 Bantams who ate 1 & 1/2 square sandcastle buckets full (made up of about 65% Layers pellets, 25% mixed corn and 10% grit.) They got the odd greenery thrown in as and when with a little bread & fruit.
My current 12 hybrids eat the same amount of food as their predecessors did... and produce loads more waste than they ever did. :o :o :o The bright side being that I now get very large eggs, and loads more chicken manure. So technically I guess, you could say it comes down to what you want them to produce for you. ::) ::) ::)