Because every login requires a session, this parameter effectively determines the maximum number of concurrent users in the system. You should always set this parameter explicitly to a value equivalent to your estimate of the maximum number of concurrent users, plus the number of background processes, plus approximately 10% for recursive sessions.

Oracle uses the default value of this parameter as its minimum. Values between 1 and the default do not trigger errors, but Oracle ignores them and uses the default instead.

The default values of the ENQUEUE_RESOURCES and TRANSACTIONS parameters are derived from SESSIONS. Therefore, if you increase the value of SESSIONS, you should consider whether to adjust the values of ENQUEUE_RESOURCES and TRANSACTIONS as well. (Note that ENQUEUE_RESOURCES is obsolete as of Oracle Database 10g release 2 (10.2).)

In a shared server environment, the value of PROCESSES can be quite small. Therefore, Oracle recommends that you adjust the value of SESSIONS to approximately 1.1 * total number of connections.

For a CDB, the root container's SESSIONS parameter specifies the total number of sessions for the database.

The SESSIONS parameter for a PDB specifies the total number of sessions that can be used by that PDB. Its value defaults to the root container's SESSIONS value. If the PDB tries to use more sessions than configured by its SESSIONS parameter, an ORA-00018 error message is generated. For PDBs, the SESSIONS parameter does not count recursive sessions and hence does not require the 10% adjustment.

The SESSIONS parameter for a PDB can only be modified by the PDB. It cannot be set higher than the CDB's SESSIONS value.