How do I track if a user is logged in on multiple devices and computers?
Check Session Limit module.

Session Limit allows administrators to limit the number of simultaneous sessions per user.
Max session is configurable, no database tables needed.
By default, a session is created for each browser that a user uses to log in. This module will force the user to log out any extra sessions after they exceed the administrator-defined maximum.
Assuming the session limit is 1, if a user is logged in to a Drupal site from their work computer and they log in from their home computer, they would be forced to either log off the work computer session, or abort their new login from home.