It depends on how you intend to use the fruits. Since you are dealing with a large group of people, it is easier to consider the amounts needed in terms of weight rather than individual pieces for grapes and strawberries. Because a significant portion of the weight of a banana includes the peel, consider 1/2 banana per person. If you are planning to make a fruit tray or a fruit salad, a good rule of thumb is consider a 4 ounce serving per person for the strawberries and grapes. If you are planning to feed 50 people, you would need about 4 ounces per person or 12.5 pounds of grapes and strawberries.