One billion! and here is the evidence that I found online :
Ward's reports that there are 1.3 people for every car in the U.S., while in China there are 6.75 people per vehicle. If China were to have as many cars per capita as the U.S., its fleet alone would amount to approximately one billion cars.
At this particular moment in time? It's impossible to tell, you can have estimate but that doesn't appear to be what you're looking for, there will be cars that aren't known to exist, cars get made and destroyed all the time so, I couldn't say. Sorry, but I hope this helps :)
