V$License: Query the oracle license, current sessions and maximum sessions 
V$LICENSE Data Dictionary View

V$license view allows you to query the current limits of all of the license settings, the current number of sessions, and the maximum number of concurrent sessions for the instance. 

This information can be used to determine if you need to upgrade your Oracle license to allow more concurrent sessions or named users: 

SELECT sessions_max s_max,
   sessions_warning s_warning,
   sessions_current s_current,
   sessions_highwater s_high,
   users_max
   FROM v$license;

S_MAX    S_WARNING   S_CURRENT    S_HIGH    USERS_MAX
-------------------------------------------------------
100      80          65          82        50

Note: Session high water mark to the database is written in Oracle Alert file when the database shutdown.