I don't think there's a limit . . . we took 4 suitcases on with us for our week-long cruise last year, as I didn't want to risk leaving anything in the car!
If you are returning to Disney they will hold it for you. Not sure if you are not returning to a Disney resort.
We have always left luggage in car boot no problems but due to increase car rental fees and port parking charges last year we returned the car to Budget at Port Canaveral and got another on return, saved about £200 for a weeks cruise, doing it this way we had to take 25 nights luggage onboard the ship. Not a problem as the beds lift to store the cases.