SELECT name, value
  FROM v$parameter
 WHERE name = 'sessions'

SELECT COUNT(*)
  FROM v$session

SELECT
  'Currently, '
  || (SELECT COUNT(*) FROM V$SESSION)
  || ' out of '
  || DECODE(VL.SESSIONS_MAX,0,'unlimited',VL.SESSIONS_MAX)
  || ' connections are used.' AS USAGE_MESSAGE
FROM
  V$LICENSE VL

SELECT
  'Currently, '
  || (SELECT COUNT(*) FROM V$SESSION)
  || ' out of '
  || VP.VALUE
  || ' connections are used.' AS USAGE_MESSAGE
FROM
  V$PARAMETER VP
WHERE VP.NAME = 'sessions'

select current_utilization, limit_value
    from v$resource_limit
    where resource_name='sessions';

select resource_name, current_utilization, max_utilization, limit_value
    from v$resource_limit
    where resource_name in ('sessions', 'processes');

select resource_name, current_utilization, max_utilization, limit_value
    from v$resource_limit
    where resource_name in ('sessions', 'processes');

SQL> show parameter sessions

SQL> show parameter sessions

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
java_max_sessionspace_size           integer     0
java_soft_sessionspace_limit         integer     0
license_max_sessions                 integer     0
license_sessions_warning             integer     0
sessions                             integer     248
shared_server_sessions               integer

select count(*),sum(decode(status, 'ACTIVE',1,0)) from v$session where type= 'USER'