ORA-00304: requested instance_number is busy

The Oracle docs note this on the ora-00304 error:

ORA-00304: requested INSTANCE_NUMBER is busy
 Cause: An instance tried to start by using a value of the initialization parameter INSTANCE_NUMBER that is already in use.
 Action: Either a) specify another INSTANCE_NUMBER, b) shut down the running instance with this number c) wait for instance recovery to complete on the instance with this number.

You will see ORA 00304 if an INSTANCE_NUMBER parameter value is being used at the time of initializing an Oracle instance
To resolve ORA-00304, you may try one of the three options:

1. Use a different INSTANCE_NUMBER
   In the initialization file, modify the initialization of the INSTANCE_NUMBER
2. Shut down the instance number causing the ORA-00304 error
3. On the instance with the ORA-00304, allow recovery to conclude