This is what I do when I cater.
Off hand, I would say 3 bags of 3 lbs of salad for 65 people. It really depends on what other foods you have. I usually go with 1-2.5 oz of salad per person. If it is just one entree with trimmings I serve 2 oz. On the average I go with 1.5 oz which yeilds between 1/2 -2/3 cup per person.
Consider too, if you are going to add tomatoes, onions, green peppers,croutons ect which can add as much as 1-1.5 oz more to the salad, if so definitely go with no more than 1.5 oz.
Even though some people will not eat salad, others will take double. It usually evens out. Hope this helps.