I agree there are a lot of garages. I think the OP wanted a hotel on the autostrada next to public transportation into the city, so that he could park for free at the hotel, and get into the city from there. I assume by autostrada he means the tangenziale. The garages in the city generally charge an hourly rate, or a daily rate. It's still not clear why he would want to pay for a car for two days that he is not going to be driving, plus paying for transportation into the city each day, when it will be much cheaper and ten times more convenient to take a train in from Naples and rent the car when leaving Rome instead of beforehand, then leaving it parked in the city in a garage, or outside of the city at a hotel for two days.