I always carry 2-4 bungees per rack (I often run front and rear racks on my bikes). Usually two bungees will secure anything, as long as you make sure to stetch them out well so they have little give left in them by winding them around the rack however much you need to depending on the load. Used to carry multiple bankers boxes of legal documents this way when I was a courier. IIRC those are 40 or 48 pounds each. I never have any trouble with my loads as long as I use at least two bungees and run them in a way that takes up almost all of the slack in them. These days I use bungees in the summer to secure all my camping gear to my rear rack when doing bike camping and the ocassional load home from the store the rest of the year.

I like bungees because they are quick and easy to get on and off. I also carry a bungee net for each rack but only use it to secure very light loads. Get bungees that aren't the black rubber ones, and make sure you get ones with plastic hooks. If they slip out of your hands while securing them the metal hooked ones can be dangerous!