Each subscription comes with 3 users. This means that you can have 3 people using each subject at any one time. Each time you launch a subject, this will count as one session. Therefore, if you launch the same subject again, it will register 2 users; if you keep clicking it, the user count will fill up, so it is important to only click once.
NB: It is recommended that you use the Cross button at the end of each session to log out as just closing the browser tab can cause a delay before the session is released.
Sometimes users forget to close a session and then move to another computer where they see the session count is exceeded. To check how many sessions you have open, first check the Gatekeeper to see how many sessions are active. If it is showing 3 of 3 (or equivalent if you have more licenced users) then click your name to open the user profile and settings panel.
Click the Sessions tab to see how many active sessions there are. Your sessions are indicated by the End Session links in the final column. Click End Session to close a session.