If you are using shared server architecture or if the DISPATCHERS parameter is set such that the total number of dispatchers is more than 0, then the default value is 1.
Range of values
The value of this parameter should be less than MAX_SHARED_SERVERS. If it is greater than or equal to MAX_SHARED_SERVERS, then the number of servers will not be self-tuned but will remain constant, as specified by SHARED_SERVERS.
SHARED_SERVERS specifies the number of server processes that you want to create when an instance is started. If system load decreases, then this minimum number of servers is maintained. Therefore, you should take care not to set SHARED_SERVERS too high at system startup.