How many KITTs were destroyed during production of the '80s Knight Rider series?

This one is actually surprisingly hard to answer, mainly because the show actually had four different grades of cars that were used in production; "Hero cars", "Stunt cars", "Jump cars" and "Shells".

Stunt / Jump cars

According to the book "Knight Rider Legacy" (referenced here in '15 facts you might not know about Knight Rider'), the show went through an average of just under one real car every two to three shows. These stunt cars were the ones that were seen driving around corners, high-speed acceleration and performing handbrake turns.

The stunt work was hard on the cars, and the show typically ruined four to nine each season. The Trans Ams were sold by GM to the producers for $1 each. Each one cost about $18,000 to modify into K.I.T.T.

This interview with Bob Ewing, the show's Associate Producer indicates that the show had two dedicated "jump cars" that could perform jump stunts without being destroyed as well as 12 other stunt cars that were dedicated to other kinds of driving, jump and landing stunts (as well as being shot at or having parts ripped off)

We have two jump cars," says Ewing. These cars look identical to K.I.T.T., but are made of lightweight fiberglass and contain high-powered engines. A stunt driver races this fiberglass car at high speed toward a hidden ramp. That ramp sends the car up into the air, then down to another hidden ramp angled for a safe landing. The car leaps over real trains and trucks. The stunts are timed to the split-second to make certain no one is in danger.

"We've never had an accident," Ewing says proudly. If a stunt is too dangerous, we don't do it."

How does the driver survive? Jump cars can be specially built to survive leaps - but why doesn't the driver get thrown out of the car by the impact? That's easy, says Ewing: "Our stuntman is tied into that car every which way but Sunday."

Shells

I've seen numerous references on the internet that they also managed to trash at least 2-3 "shells" (mostly old trans-ams that the producers would buy from local car dealerships and modify by replacing panels (to create the unique shape) and removing the engines to lighten them enough to fire them from an air-ram) per show but I've struggled to find a formal interview confirming it.

Hero Props

There were also a set of around six "Hero" cars that were used for close-up filming during the various seasons. According to this interview with Hotrod.com, the three original "hero" cars supplied by Pontiac were supposed to be destroyed at the end of the first season but somehow managed to survive the crusher and ended up in private hands.