Ok, so increasing a recipe originally for 16 to 24, you need to figure out how many times larger you need the recipe, so divide 24 by 16, and you get 1.5 (one and a half times larger)
You then just need to take each ingredient amount and multiply it by 1.5
2 cans chopped clams x 1.5 = 3 cans
4 cups broth x 1.5 = 6 cups
6 cups milk x 1.5 = 9 cups
4 cups cubed potatoes x 1.5 = 6 cups
Does this make sense?