You can hook up as many as you can fit on the mobo.....some mobos have 2 IDE's and up to 4 or more sata ports which would allow for 8 Hard drives right there.....of course then you wouldn't have room for a CD-ROM drive so you couldn't install windows
Well..you could install Windows...but you'd just have to improvise by temporarily disconnecting any uneeded drives, and leaving the specific Hard Drive you want to install an OS on, connected...
You can hook up as many as you can fit on the mobo.....some mobos have 2 IDE's and up to 4 or more sata ports which would allow for 8 Hard drives right there.....of course then you wouldn't have room for a CD-ROM drive so you couldn't install windows