When I was working at AOL, and we were architecting 
low-latency/high-throughput systems, one of the rules-of-thumb that 
we were given by the data/filesystems experts (including the key guy 
from the team that built the world-renowned CLARiiON RAID array) was 
that you should have no more than four SCSI disk devices per channel. 
Indeed, we were told that this has pretty much always been the rule, 
because as disk devices have gotten faster, so has SCSI.

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A factor that totally invalidates that statement is the fact that
fiber-channel disks "look" like scsi disks to NetBSD.  We have
installations with many, many disks on a fiber channel.  In fact, the
problem we ran into was that the isp driver didn't allow enough LUNs for
all the disks.  (We hacked that locally.)

Ken