As they said, get in to see your doctor immediately. You most likely have a mulfunction in your device. It was sensing something that it thought was a shockable rhythm, but it most likely wasn't... since the first 20 shocks didn't terminate it. They can interrogate your device and find out what it was sensing.
I agree - you should have it checked out immediately. When my Dad's ICD went off twice in one week, the cardio had him come right in. The goal is no more than twice a month and hopefully, with the right medications, even less than that.
An ICD is meant to detect an abnormal heart beat and covert it back to a normal one....it going off 21 times in a row just doesn't sound right to me. I would make the call to the doc that implanted it or your personal heart doc and let them know right now...it is obviously detecting something that it is having difficulty correcting and they may want to have you seen just to rule that out. I would call now if it is happening now or within the last couple of hours...don't wait till their office opens....good luck