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It depends to the queue_depth. Rules per VSCSI:

Maximum disks per adapter = ( 512 – 2 ) / ( 3 + queue depth )

If you keep the default queue_depth to 3, the you can map a maximum of 85 LUN per VSCSI or pair if you use dual VIOS.
For example if you change the default queue_depth to 20, the you can map a maximum of 23 LUN per VSCSI or pair if you use dual VIOS.

$ chdev -l hdisk0 -a queue_depth=20 -P
$ chdev -l vscsi0 -a vscsi_path_to=30 -P
$ chdev -l vscsi0 -a vscsi_err_recov=fast_fail -P

http://www-03.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/TD105745

http://capacityreports.com/VIO/vio_tips.html