Oracle - View available and used database sessions

-- The number of sessions the database was configured to allow
SELECT name, VALUE
FROM v$parameter
WHERE name = 'sessions'
;

-- The number of sessions currently active
SELECT COUNT(*)
FROM v$session
;

-- Source: http://stackoverflow.com/questions/162255/how-to-check-the-maximum-number-of-allowed-connections-to-an-oracle-database