I just got an email from a friend who is not a forum member, but sometimes checks out the forum as a guest. He's pretty knowledgeable on this topic, and he said it would be ok if I shared his email to me here:
This one isn’t even close. As long as the question regards a single vehicle platform It’s the Lamborghini Countach. Christian's statement that there are more kits of this car than Tri-5s, Deuces, or anything else combined is spot on. If you were willing to consider just the nameplate, the Ford Mustang would be the winner, but you’d have to count all the (very different) generations of the car which weren’t based on the same platform, and the way I read it that goes against the spirit of the thread.
Just in terms of scales, Countachs has been done in plastic, that I’m aware of, in 1/64, 1/43, 1/40, 1/38, 1/32, 1/24, 1/20, 1/16, and 1/12. Probably more. That’s not counting several non-scale models (say, Bandai mini 4wd versions) or the myriad diecasts that have also been available in kit form.
There are at least 3 completely different toolings of the car in 1/16, for instance, maybe 5-8 in 1/20, and in 1/24 there are several dozen. The car hit at exactly the right time, and since it was in production for a long time (1974-89), had many different distinct versions to kit, and the Japanese companies all had to have one in the motorized era, then the car remained popular when people wanted full-detail kits it got re-kitted in the 80s many, many times.