We are using DB2 V8.2 ESE on AIX (64-bits) and would like to know how to
determine the values for the NUM_IOSERVERS. According to DB2 manual or
technotes, they stated that it is advised to set the NUM_IOSERVERS equal to
the number of physical disks that the database resides on plus 1 or 2.

In my case, it is a SAN environment (ESS), so a ESS logical disk appear to
AIX as a physical disk, but in fact are logical storage allocated across
multiple physical disks in an ESS array.

So, under this kind of environment, when I set the NUM_IOSERVERS, should I
still count the number of disks appear on the AIX level or how many physical disks on the ESS array?

If you are using a disk subsystem that has striped arrays (regardless of exact type of stripping) then I would recommend setting NUM_IOSERVERS to the number of CPU's.

For high-end SAN or NAS systems, I would also issue the following dbm command for the instance:
db2set DB2_PARALLEL_IO=*

let's say there are 8 dbps per host and 8 cpu's on a host. As wongpcl mentioned, the Aix "disks' are striped across 8 SAN disks. What would you recommend in this case for a v8.1 system?