If you tell me the model of the motherboard, I could look it up.
There are two kinds of hard drive interface in common usage. IDE is old and being phased out. It uses a 40-pin ribbon cable and allows two drives to be attached to one port one the motherboard. One drive must be configured as Master, and one as Slave. In California, these must be referred to as Primary and Secondary by state law. The cable looks like this: http://www.wpclipart.com/computer/hardware/cables/ide_cable.png.html
, or, in rare cases, like this: http://www.endpcnoise.com/cgi-bin/e/std/sku=blackroundide
The more modern, and faster, drives use the SATA interface. The cable is heavier and only has seven pins, and only one can be connected to each port on the motherboard. The cable looks like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16812719003&cm_re=sata_cable-_-12-719-003-_-Product
, although the connectors at the end may have a right-angle bend.
So your motherboard can accomodate two IDE drives per IDE port, and one SATA drive per SATA port. If I knew what the motherboard model is, I could look up how many ports it has of what type.
You could always buy an add-in controller to run more drives if you need to.