Let's quote Kellerman. Here he is on the "flurry of shells":
Mr. SPECTER. Now, in your prior testimony you described a flurry of shells into the car. How many shots did you hear after the first noise which you described as sounding like a firecracker?
Mr. KELLERMAN. Mr. Specter, these shells came in all together.
Mr. SPECTER. Are you able to say how many you heard?
Mr. KELLERMAN. I am going to say two, and it was like a double bang--bang, bang.
I would think that a bullet fragment hitting the chrome over the windshield and another fragment hitting the windshield would sound like a "flurry" of shots/shells. Plus, as he later said he was splattered with blood from the wound to JFK's head. Which wouldn't have happened if the wound blew out the
back of JFK's head and not the top/right. But that's another story.
In any case, he says the "flurry of shells" was "two."
Link/source:
https://www.jfk-assassination.net/russ/testimony/kellerma.htm