Strictly speaking, this is up to you. The sugar helps to macerate the berries - encouraging them to release their juices as they heat. The amount you add is really up to you and may depend on the ripeness/sweetness of your fruits. As this is a simple syrup, strictly speaking, you're looking to keep things at a ratio of 1 part sugar to 1 part water - this results in the rich sweet syrup style used in cocktails and mocktails.