Only eight rental cars in arcade mode. Polyphony should keep up with its competitors (like Forza, Assetto Corsa and Project CARS) since in their respective arcade/free play modes, you can drive almost anything they offer right off the bat.
In GT Sport's case, if they were to implement all existing (and future) cars in the game in arcade mode, this comes at the expense of tuning some aspects of the cars and adding your custom liveries to them. Just like in Forza, they should also keep the ability to use garage cars as well.