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.