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Limit the number of sessions concurrently connected to a database

     Oracle helps you ensure that your site complies with its Oracle Server license agreement.If your site is licensed by concurrent usage,you can track and limit the number of sessions concurrently connected to a database.If your site is license by named users,you can limit the number of named users created in a database.In either case,you control the licensing facilites,and must enable the facilities and set the appropriate limits.
     To use the licensing facility,you need to know which type of licensing agreement your site has,and what the maximum number of sessions or named users is.Your site may use either type of licensing,but not both.
     However,most site leave the licensing mechanism disabled by omitting
        LICENSE_MAX_SESSION
        LICENSE_SESSIONS_WARNING
        LICENSE_MAX_USERS
from the parameter file, or set the value of all three to 0.
you can check it use Oracle Session Manager (Database->Info,in the LICENSE section)
or use the following statement:
select sessions_max,sessons_warning,session_current,session_highwater,users_max
from v$license;

     Limit the number of sessions that can be connected simultaneously to the database on the specified computer by using concurrent usage licensing maybe useful when a production database is too busy to deal with many sessions.Although it is not reasonable usage.
     You can set a limit on the number of concurrent sessions before you start an instance,or change the maximum number of concurrent sessions while the database is running. (Oracle session manager can help you do this on the fly).
     After your instance's session limit is reached,only users with RESTIRCTED SESSION privilege( usually DBAs) can connect to the database.When a user with RESTIRCTED SESSION privilege connects, Oracle writes a message to the ALERT file.
     Use the following statement to change LICENSE_MAX_SESSION
        Alter system set LICENSE_MAX_SESSION = 64 //whatever value you like :)

     In addition to enforcing the concurrent usage limits,Oracle tracks the highest number of concurrent sessions for each instance which names "high water mark".

NOTE: Oracle asks you connect only to terminate unneeded process.Do not raise the licensing limits unless you have upgraded you Oracle license agreement.