Prota OS is free and can be downloaded here if you want to install it on your Raspberry Pi.
We are supporting a free Raspberry Pi version as we believe that the community of makers has the capacity and passion to shape what's going to be the next big thing in the IoT universe.
This OS will therefore remain free. We will in the near future add the possibility for a small subscription to get additional features and apps, leaving of course the choice to keep the free version only.
You can use Prota OS on as many devices as you want.