Do not add the fruit during the boil!!! You will release all of the fruit pectin and make a very cloudy product. It is usually best to add the fruit during the ferment. 20 lbs. of fruit for 10 ten gallons is in the ballpark for the proper amount of fruit. I made a 10 gal. batch of cherry stout and used 18 lbs of cherries. It had a good fruit flavor.