There are 36 cars in the BETA. (then you get dupes) Daily prize car can be reset with time zone change.
I will try my best to explain how to do it - it's easy really.
It's 00:10 5th of April - you get the prize car
Without exiting GTS go to PS4 settings and change the time zone (not the clock just the time zone) in order to be on 4th of April 23:10
Go back to GTS and enter Sport mode then back to main menu - in game calendar should be on 4th again
Exit the game and wait till 00:01
Now start the game and you should get another prize car (no driving required)
Repeat
NOTES:
Changing time zone back and forth wont work - the clock in game has to move from one day to another by itself.
Playing the game while doing this will require doing the marathon. (or not ? test yourself)
Your welcome