As of April 17th, 2015500 to 1. A few couple of days later, on April 22nd, 2015, the Orange variant of the Mazda Mazdaspeed 3 was released for 0 making car slots virtually free as they are included for every car purchase the player makes and can be sold afterwards.
As of December 13th, 2012
With the patch of December 13th, the level cap was raised to 60. At level 60, a player would be eventually rewarded with one additional car slot.
As of August 22, 2012
When the car classes patch was released, players were able to unlock more slots for free. Players begin with two car slots and additional car slots are awarded at levels 12, 20, 30, 40 and 50. Extra car slots are still given when SpeedBoost cars are purchased or for 500 SB.
Before August 22, 2012
Each car in a player's garage requires a car slot. Two slots are available from the start while one extra slot is unlocked at Level 12. More car slots could be purchased for 500 SpeedBoost. Extra car slots were given when SpeedBoost cars were purchased.
If a car is sold, the car slot remains, allowing a new car to be purchased in the same slot.
During the closed beta sessions, there was no car slot limit and no SpeedBoost purchase was required. Car slot limits began during Closed Beta 4, on May 14, 2010. Until patch 5.03, players could have more than five car slots. After 5.03, there was no limit on the size of players garages.