hi, you could enable auditing and audit successful logins
( not recommended for high volume connections),, typically
we audit failed login attempts to monitor if anyone is hacking.
If you need to see how many concurrent connections, then you
can query v$process, v$session, etc.
If your new to Oracle , then OEM dbconsole may be a better alternative
( db management install options).
Ora10gR2 Docs: www oracle.com/pls/db102/homepage