V$ACTIVE_INSTANCES

V$ACTIVE_INSTANCES displays the mapping between instance names and instance numbers for all instances that have the database currently mounted.

Column	Datatype	Description
INST_NUMBER	NUMBER	Instance number
INST_NAME	VARCHAR2(60)	Instance name

Note:

1. This view is ONLY populated in Real Application Clusters mode (or in Parallel Server mode in Oracle8i and earlier).

Oracle data dictionary views

Oracle dynamic performance views