Code:
cat /u01/app/oracle/product/9.2.0/network/admin/listener.ora 

# LISTENER.ORA Network Configuration File: /u01/app/oracle/product/9.2.0/network/admin/listener.ora
# Generated by Oracle configuration tools.

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC))
      )
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = TCP)(HOST = rac1)(PORT = 6969))
      )
    )
  )

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = PLSExtProc)
      (ORACLE_HOME = /u01/app/oracle/product/9.2.0)
      (PROGRAM = extproc)
    )
    (SID_DESC =
      (GLOBAL_DBNAME = lnx920)
      (ORACLE_HOME = /u01/app/oracle/product/9.2.0)
      (SID_NAME = lnx920)
    )
    (SID_DESC =
      (GLOBAL_DBNAME = rman920)
      (ORACLE_HOME = /u01/app/oracle/product/9.2.0)
      (SID_NAME = rman920)
    )
    (SID_DESC =
      (GLOBAL_DBNAME = rmandg)
      (ORACLE_HOME = /u01/app/oracle/product/9.2.0)
      (SID_NAME = rmandg)
    )
  )
just edit listener.ora and start the listener, the sample I pasted is listening for 4 services (databases)