There are a few different limits that might come in to play in determining the number of connections an Oracle database supports. The simplest approach would be to use the SESSIONS parameter and V$SESSION, i.e.
The number of sessions the database was configured to allow
SELECT name, value
FROM v$parameter
WHERE name = 'sessions'
The number of sessions currently active
SELECT COUNT(*)
FROM v$session
As I said, though, there are other potential limits both at the database level and at the operating system level and depending on whether shared server has been configured. If shared server is ignored, you may well hit the limit of the PROCESSES parameter before you hit the limit of the SESSIONS parameter. And you may hit operating system limits because each session requires a certain amount of RAM.
From:
http://stackoverflow.com/