ORA-00018: Maximum number of sessions exceeded

What does it mean?

The database has reached the maximum number of sessions allowed by the SESSIONS parameter; no new session can be opened.

Why does it happen?

SESSIONS/PROCESSES set below the workload's needs, an oversized connection pool configuration, a connection leak, or accumulation of sessions that are never closed.

How to fix it

View the current session count: SELECT COUNT(*) FROM v$session. Check the limits: SELECT * FROM v$resource_limit WHERE resource_name IN ('sessions','processes'). Increase PROCESSES and, indirectly, SESSIONS (via SPFILE, restart required): ALTER SYSTEM SET PROCESSES = 500 SCOPE=SPFILE. Review connection pool sizes and leaks in the application.