ORA-00020 maximum number of processes exceeded

ORA-00020 maximum number of processes exceeded
Cause: All process state objects are in use.
Action: Increase the value of the PROCESSES initialization parameter.

ORA-00020 comes under "Oracle Database Server Messages". These messages are generated by the Oracle database server when running any Oracle program.

Reference: Oracle Documentation 

How to increase PROCESSES initialization parameter:

1.    Login as sysdba
    sqlplus / as sysdba
    
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