As you progress and get better at the game, you will start to earn more bonus shots as you learn how each shot type works and how to get the most effect score ratio out of them.
One way to earn multiple bonuses would be to play challenge area 2 on stage 2. You can earn up to 10+ bonus icons just for completing the course. With this method, you can use the split shot. When you fire the shot, line up as many objects as you can and cause complete destruction. The video is just an example of how to earn multiple bonus icons with a single shot, but the further you get into the game, this will become more regular.