Put a high upper limit only. Don’t touch the bottomline.
For example, no more than 4 cars per person: Average Joe won’t even know this rule exists but it will still reduce mineral mining due to people who collect cars.
Possible problems with my shitty example: Now a car is a controlled substance. Who decides the limit and how? What if there is a mental disease (with a better example this would make more sense) which requires a person to have 20 cars?