There is no officially documented maximum for the number of logged in users in TPAM.
The emphasis would be on the activities and tasks that the users are performing. Some users would be consuming more resources than others and therefore, the calculation becomes more complicated. One user running a backup, importing a large batch file, or starting a PSM Session would use more resources than a user who is idle or simply browsing the web interface.
The total number of users is less important than the total resource usage of the appliance.