How many drives depends on how many ports the motherboard has.
IDE (PATA, paralell) ports are now less common and most motherboards only have one which can take two devices and is now most commonly used to take a pair of optical drives.
And most motherboards now sport on average six to eight PATA ports which can take both hard drives and optical drives.
So, there's no set number, depends on the motherboard.
An example:
Six SATA ports, 1 IDE port.
The user decides to have a DVD Blu Ray drive and a DVRW drive on two SATA ports and four hard drives on the other four SATA ports.
That's kinda typical.
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PSD99: Button it.
MadX: don't stir things.
I'll be keeping an eye with my finger poised on 'delete'.