"Apple grew from 3 to 45 between April 2017 and March 2018 and now this number has increased to 55," the report added.
Behind Apple is Alphabet's Waymo which currently has 51 self-driving vehicles and Elon Musk-owned Tesla which has 39 cars.
Currently, the California Department of Motor Vehicles (DMV) has provided permits to 53 companies for self-driving cars that include safety drivers, resulting in a total of 409 vehicles and 1,573 safety drivers in the US.
As of now, the DMV has not issued any permits for complete driverless testing because for that to happen, companies must have to have a pre-testing of the vehicles in controlled conditions.
"The DMV is currently reviewing two driverless testing permit applications," a DMV spokesperson reportedly told TechCrunch.
Code-named "Project Titan", Apple's self-driving programme's fleet is reportedly made up of Lexus RX450h SUVs, each of which is fitted with huge racks of LIDAR and radar sensors as well as cameras.