ORA-00020: maximum number of processes (500) exceeded

How to increase PROCESSES initialization parameter:

1. Login as sysdba
sqlplus / as sysdba

or

sqlplus /nolog

SQL*Plus: Release 11.2.0.3.0 Production on Wed Oct 4 11:32:29 2017
Copyright (c) 1982, 2011, Oracle. All rights reserved.

SQL> connect sys/oracle as sysdba

Connected.

2. Check Current Setting of Parameters
sql> show parameter sessions
sql> show parameter processes
sql> show parameter transactions

3. If you are planning to increase "PROCESSES" parameter you should also plan to increase "sessions and "transactions" parameters
A basic formula for determining these parameter values is as follows:

processes=x
sessions=x*1.1+5
transactions=sessions*1.1

4. These paramters can't be modified in memory. You have to modify the spfile only (scope=spfile) and bounce the instance.
sql> alter system set processes=500 scope=spfile;
sql> alter system set sessions=555 scope=spfile;
sql> alter system set transactions=610 scope=spfile;
sql> shutdown abort
sql> startup