How to check the capacity of your instance - 10g and higher

SELECT 'Max Sessions - '||highwater "Resource"

  FROM DBA_HIGH_WATER_MARK_STATISTICS

WHERE name = 'SESSIONS'

UNION

SELECT 'Processes - '||value

  FROM v$parameter

WHERE name = 'processes'

UNION

SELECT 'Sessions - '||value

  FROM v$parameter

WHERE name = 'sessions';

Resource
----------------------------
Max Sessions - 127
Processes - 300
Sessions - 335