The coverage provided by a credit card company for a rental car is typically not going to be sufficient. A credit card company will provide coverage for damage to the rental car, but it won’t cover you if you injure someone in an accident and they sue you. The primary policy should be your own auto insurance, especially because it provides liability coverage if you injure someone in an accident.
If you do not have auto insurance because you do not own a car, that is a different scenario. In this situation, you should purchase a full-coverage liability plan through the rental car agency to make sure you are covered in case of an accident. The rental car insurance offered by your credit card company would serve as a secondary damage policy.
If you want to use your rental car insurance through your credit card, be sure to pay for your rental with that particular card. Also, make sure you know the details of how your credit card works in terms of car rentals. Every card is different. Happy trails!