First, to clarify there are no “Pro accounts”. The Pro version is per device and local to just the device it was purchased on. It doesn’t link to a Commander account at all. This is because Commander accounts are not required since the app could be used via texting only if you want.
However, the Elite version is for an account. You must have a Commander account and it will apply to all devices on that account.
Now to answer the question: the free version allows 3 devices on a Commander account. If at least one device on your account has Pro then you can have 10 total devices. If you upgrade to an Elite account you can also have up to 10 devices.