I’ve been unable to verify yet, but it is possible this functionality was never implemented with using steam online subsystem, (just like “get ping”).
The current workaround I’ve found is to use the “Advanced Sessions” plugin and use the attachable session “extra settings” (which can be updated and retrieved) to create a custom system for keeping track of and updating the session player count that way.
It would be better to be able to retrieve the session information directly tho as I believe it would be a more stable solution.