This may sound pretty basic to some, but to a beginners I think the following comments will be helpful.  How do you know how much food to bring.  Before you leave write a menu for breakfast, lunch and dinner for each day.  If you plan on instant oatmeal for breakfast, figure out how many packets each person will eat and for how many days.  I figure 3 packs per person per breakfast.  If you are having pancakes again calculate how many pancakes each person will eat.  Multiple by the number of people and how many meals you plan on eating pancakes.  We generally eat sandwiches for lunch.  Often summer sausage and cheese.  I figure three slices of bread per person.  One slice of cheese per sandwich and one inch of sausage per sandwich.  One inch of sausage is 4 slices one quarter inch thick.  If you bring summer sausage, buy small sausage so you don't have to worry about spoilage once you open the package.  We figure on fish one meal a day and a package of pasta which you can buy at any grocery store.  Just read the package for serving size.  Our primary vice is bringing cookies.  Again count how many cookies you want to eat per day and multiple by the number of days.  I once suggested to my son that we could reduce weight if we didn't bring cookies.  He thought that was a bad idea.

My basic point is use a pencil and paper and calculate how much to bring.