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.