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

SQL> show parameter processes

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
processes integer 300

SQL> alter system set processes=600 scope=spfile;

System altered.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.

SQL> startup
ORACLE instance started.

SQL> show parameter processes

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
processes integer 600