ORA-00059: maximum number of DB_FILES exceeded

alter system set db_files=300 scope=spfile;
shutdown immediate
startup


SQL> show parameter db_files

NAME     TYPE VALUE
------------------------------------ ----------- ------------------------------
db_files     integer 200
SQL> alter system set db_files=300 scope=spfile;

System altered.

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

Total System Global Area  570425344 bytes
Fixed Size    2097888 bytes
Variable Size  205524256 bytes
Database Buffers  356515840 bytes
Redo Buffers    6287360 bytes
Database mounted.
Database opened.
SQL> show parameter db_files

NAME     TYPE VALUE
------------------------------------ ----------- ------------------------------
db_files     integer 300