How many hard disks will one motherboard support?
It depends on the type interface you would be using, IDE, SATA,
SCSI (or a combination of these).
Generally, most motherboards use IDE (or UIDE), which has two
IDE Ports each of which supports two drives (Master and Slave), be
they hard disks, CD or DVD drives.
So the answer is four.
However, a type of motherboard known as "RAID" which is short
for Redundant Array of Independent (or Inexpensive) Disks, can have
double that number. These are used for servers, but can be used for
personal computers although not normally necessary, as they do not
function in quite the same as the conventional IDE arrangement.