If you look up previous threads on this topic, fellow Lakeside-owner, you'll find that many experienced campers on this site don't appear to bother too much with actual freezer blocks. Using large, plastic drinks bottles filled with water or cordial, frozen and placed on top of the food is one method.
The advantage of freezer blocks over frozen drinks is that a lot of sites either offer a freezer to refreeze them or an exchange system (you give them yours and get the same amount of frozen ones back). If you are going for more than a few days, and the site offers this, you would be better with freezer blocks as your drinks aren't going to stay frozen for a week. I'd think 4 large ones would do. I just tend to take all of mine. The other thing we do is to take a small lunchbox sized coolbox for the days when we have only got a pint of milk.