There are 123 Jack Spots placed throughout the city. Although there are only 41 cars, each has 3 separate Jack Spots from which you can change your car. This includes the 10 Most Wanted cars although you can’t find these ones without first owning the cars. Also, the Jack Spots that relate to the current car you are driving will not show on the in game map.
To check which Jack spots you have already found, press to bring up the map and then or to toggle through until you reach the Jack spots.
All Jack Spot locations can be seen below: