Going over things, it should be relatively easy to know how many weapons were used:
1- Trace down all those injured and get bullet fragments from them. I believe the weapon can be traced (the pistol is in police custody). If all the bullets came from the same weapon, then you can make some judgement.
@RescueRanger can elaborate more on the process.
2- Check and see how many bullets are left in the magazine of the pistol. I doubt the guy had enough time to reload.
3- Match all the shells found from the crime scene and then roughly match it with the gunshot sounds heard on the video.