By nature, fresh raspberries, especially when at their ripest, are a juicy fruit. This natural juice adds extra moisture to the batter, and putting in too many raspberries can actually hinder how the blondies bake. During recipe testing, I found that too many raspberries yielded a soft blondie that wouldn't set. So, I would stick with the suggested amount. Alternatively, you can use freeze-dried raspberry pieces and add however many you like!