Set up classes? I believe you mean objects of classes. Classes themselves cannot be "created" at run-time, unless you count templated classes, which still aren't created at run-time.
std::vector<Car*> cars; //vector of pointers, to avoid constructor trouble
std::cin >> input;
for(int i = 0; i < input;++i) //Create (input) cars
//Do something with all your (input) new cars
while(cars.size() > 0) //Delete all the cars, free the memory