The 'standard' mother board is designed to run up to 4 hdd or cd in any combination. just wondering what peoples view is on how many hdd's and cd's realisticly need to be searched for in a basic desk top OS.

I did at one time have several systems with 24 drives. That was a long time ago when drives were small and I needed 40+gig. What seems a reasonable number build an OS for today?

You have to take into consideration the different kinds of interfaces, such as SATA (with AHCI).
And there's no such thing as standard MoBo. Older MoBo will support 4 PATA disks, some newer MoBo can support 4 SATA disks and 1 PATA, some 6 SATA and 2 PATA, etc.

It really depends on the user... a standard user, be it a family guy or corporate drone, will do just fine with a single harddisk. Other people will need a lot - don't hardcode limitations if you're doing kernel coding.

Personally, for my workstation, I get along just fine with 3 internal SATA drives (one intel X25-E, and two 74gig raptors striped) and a bunch of external USB "bulk storage" drives.