|There's no such thing as hard drive slots. Hard drives go into bays - standard sized compartments in the case - or hang from bays in some cases.|
If you mean for the Dell in your specs -
"Dell Optiplex 960 mt desktop"
the answer is you have one 3.5" bay for one 3.5" hard drive, unless you're not going to be installing an optical (CD or DVD) drive, in which case you could install one more 3.5" hard drive in a 5.25" bay, by using standard mounting brackets for that purpose.
The Dell Optiplex 960 series has three possible cases:
I found no info about how many hard drives can be connected to the mboard.
I'm assuming you can connect up to 4.
Scroll down, click on Tech specs.
How many 3.5" bays your case has determines how many 3.5" hard drives you can install inside of it.
The 960MT uses the "Desktop" case.
The max is for the Tower case, the largest one - two internal 3.5" bays - although you could install one more 3.5" hard drive in a 5.25" bay if you installed only one optical drive, by using standard mounting brackets for that purpose.
The one for the 960 MT is the mid-sized "desktop" one. It can be used horizontally or vertically as a slim tower.
If you're not going to be using the 3.5" external bay (for a floppy drive or card reader, etc.) then you should be able to install a hard drive inside the case in that bay, but you might need to drill holes for the hard drive mounting screws if you use that bay.
So - the max for the (MT) Desktop case is 2 hard drives, or 3 if you don't install a CD or DVD drive.
- the max for the Tower case is 3 hard drives, or 4 if you install only one CD or DVD drive.