Sounds like an easy enough question? The answer from experienced bakers -- as much as possible. Blueberries are a true value added ingredient in muffins, and USHBC interviews with consumers indicated that consumers want to see the blueberries on the inside and out. While some bakers are afraid of blue color from the blueberries spreading to the cake -- consumers had a different view altogether. They want to see the color. After all that is why they purchase the blueberry muffin.
This "maxxed out" muffin has more blueberry than four by weight!