Oracle - Query DB Sessions SQL

I had an error today when trying to create a datasource in Weblogic. The error was:

ORA-27101: shared memory realm does not exist



Cause: Unable to locate shared memory realm


Action: Verify that the realm is accessible

The cause of this error was due to the number of database sessions being outnumbered by the number of Weblogic instances. We set up 7 environments, each with 50 maximum connections but only 100 sessions on the DB side.... as you can imagine we smashed the limit.

You can check the number of sessions using:

 SELECT name, value  
  
 FROM v$parameter 
  
 WHERE name = 'sessions'  

And you can check how many are currently in use using:

 SELECT COUNT(*)  
  
 FROM v$session