The PROCESSES parameter defines the total number of processes (foreground + background) that are allowed for the database instance.

Check

SHOW PARAMETER processes

To check whether the current limit has been reached...

SET LINESIZE 200

SELECT *

FROM v$resource_limit

WHERE resource_name = 'processes';

Change

ALTER SYSTEM SET processes=400 SCOPE=BOTH;

Implications of changing this parameter...

The minimum value of SESSIONS (and the default if it is not explicitly set) is derived from PROCESSES using the formula...

SESSIONS = (1.1 * PROCESSES) + 5