Sizing hardware drive capabilities (in the absence of probed devices)

[Don NetBSD <netbsd-embedded%gmx.com@localhost>]

How can I determine the number of /potential/ disk devices (sd(4))
that a system MIGHT support -- *if* the drives had been installed
prior to boot?  E.g., if I have a 15 slot backplane but only have
a drive installed in slot 13, then *that* appears as sd0 and there
is no mention of the potential for the other 14 drives.

Presumably, I can wire down each sd(4) device to correspond to a
particular "slot" (SATA port) in the machine when I build a kernel
with that in mind.

[This allows software to KNOW that sd0 is "the drive in the top
left slot" even if there is no drive present there when the machine
boots]

How can I configure a kernel to support a very large number of
(wired down) drives even if the hardware to support those drives
isn't present (I'm thinking about the case of having a couple
of disk shelfs which may/may not be present at any given time)?

Or, where does the probe actually take place (module name)
and I'll dig through the sources myself...