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

[jmodic]

Beacuse you can not change parameter called process by alter system

Don't be silly, of course you can change it directly in spfile!

Connected to:
Oracle Database 10g Enterprise Edition Release 10.1.0.4.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> alter system set processes=150 scope=spfile;

System altered.

SQL>