Oddbean

How can there be a no-logs policy if there is a maximum number of devices that can be logged in simultaneously per account? How does device A know that there are too many? A central server must issue a notification and log which devices are logged in.

Logging relates to what's done on the VPN and IPs that connect in a medium/long/permanent-term basis. IPs needn't be logged. Obviously servers are aware of them to accept connectivity, but they aren't logged, all data is ephemeral and processed in real-time. Regarding number of devices logged, that's fairly easily worked around. The client has to be connected to your account. That session ID can present a device count + 1 status to your account. I'm not sure how Proton specifically does it, but as someone who works in the field, there are multiple ways to have that covered.