You will probably find that the amount of fruit varies largely by type, preparation, and style of beer. It would be easier to provide sound advice if you could tell us a little bit more about the beer that you're planning to make.
Some fruit you may want to avoid the stones (e.g. cherries or peaches) and rinds could be a help or a hindrance depending on the type of fruit and the desired flavor.
I personally have experienced my best results with fruit and beer when adding the fresh fruit in a suspended muslin bag directly to the keg, however this introduces new sets of difficulties and is best for beer that will be consumed rapidly. It also, of course, require a keg setup.