This is correlative to my experiences with sauces, not as much beer, with a little beer experience. (I am an amateur saucier, or flying saucer, whichever)
For 5 gallons in order to incorporate a good flavor, I would use 2 lb of canned puree, or 4 pounds of fresh berries pureed in a processor or blender.
I can say that rasberries tend to IME throw off more flavor than some other berries, so I may would go a bit lighter, depending on the intensity you desire. Maybe 1.5 lbs. Of course if you want it really fruity, add more.