Credential Provider Usage

According to the readme file, the limit for the Credential Provider is 50 for both the demo subscription and the paid version.

What is the difference between the two? Is the difference that the demo subscription has an expiration time?

Is it 50 per-machine or per-user?

Is there a reason why it’s limited to 50 even when having a paid subscription?

the limit for the subscription of the Credential Provider depends on the level of the subscription, but is >50 in any case, so i think you misunderstood that. There is no difference between the versions, they offer full functionality, since the subscription is managed in privacyidea.
The demo limit is 50 active token in privacyidea. If you have more than 50 active token and no “real” subscription, privacyidea will refuse to communicate with the Credential Provider.