I get through a lot of computers and use ALOT of hard drives for storage. Some are external drives that work without the case (when the adapter fails). How would you go about connecting 20 hard drives (not external) to one computer. I know with external drives providing the power supply works you can get about 9+ using powered usb hubs.
Outside of the obvious issues regarding power consumption and differing interfaces, I would say if you have a tower capable of holding 20 drives and multiple PSU's then I guess multiple RAID arrays across multiple RAID controllers. UW SCSI I suppose, but then you would have to buy the proper drives. RAID 5 is normal for large sets. You are out of luck if all your drives are PATA. Although you could probably find a multiple controller card solution for IDE drives, I doubt it would be elegant. Your best bet is to look into multi-drive external enclosures providing RAID support that operates as a NAS. Or you could buy a rack mount server unit that has a similar number of drive bays. In any event all your drives would need to have the same interface.