ORA-00020: maximum number of processes (%s) exceeded

Question:  When connecting to Oracle I got this ORA-00020 error:

ORA-00020: maximum number of processes (1100) exceeded
ERROR at line 1:
ORA-01012: not logged on  

What is the cause of this ORA-00020 error and how do I fix it?

Answer:  The ORA-00020 is a serious production error because a user cannot connect. 

The ORA-00020 is caused by two things:  

1. Disconnected processes:  Rogue “zombie” connections  to Oracle that are idle (not working).  To fix this, use the ALTER SYSTEM KILL command.  You may also need to kill session at the OS level with the KILL -9 or the ORAKILL command.  
2. Too few process buckets:  Oracle limits the number of connected processes with the processes parameter, and you may get the ORA-00020 error as the natural result in growth of system usage. 

To fix this, increase the processes parameter, usually doubling the value to allow for future growth.  

The OERR command shows these details for the ORA-00020 error:

ORA-00020: maximum number of processes (string) exceeded

Cause: All process state objects are in use.  

Action: Increase the value of the PROCESSES initialization parameter

Simply increase your processes parameter and you are all set!  

See here on how to change initialization parameters.