We will never know the number of cars sold for Bitcoin. Such a huge companies does not share that kind of statistics usually. We can only know total amount of cars sold per year. Also, I dont think that a lot of car were even sold for Bitcoin. An option to buy Tesla with Bitcoin was just a strategic move do get more attention to Tesla inc. This is just marketing and Bitcoin was just an instrument in it. Just take a look for example how
tesla.com popularity changes when Elon Musk announce something connected with Bitcoin and Tesla.