The max these days is two IDE devices since IDE support has been dropped. In fact, some motherboards don't have any IDE ports at all. Also, if this is a new build, AVOID IDE at all cost. SATA is the only way to go these days.
For "New" MBs
as runswindows95 indicated 2 IDE devices, if MB has the IDE data port onboard. As on some MBs the serial port and the parallel port has gone the way of the dodo bird.
As to HDDs, DVD drives, and printers scanners
typically up to 6 to 8 HDDs and DVD drives in any combinations internally. My current I5-750 has:
2 ea SSds, 2 each HDDs, plus a Slot to insert a 2 1/2 in HDD, a Blue-ray writer, and a DVD write. Plus 4 external HDDs connectede to USB ports. NOTE: you can probably use as many printers and scanners as you can afford - in any cas More than is practical.
How about 4 printers, 2 scanners, 6 thumb drives and 10 external hDD and dvd drives.
LOL - Really the total is MORE.
The normal user will NOT max out USB devices, ie printers, scanners, Thumb drives, Keyboard, mouse, HDD?DVD USB enclosures. Usually 2 USB ports on front of computer and 2 -> 6 0n back. That is 8. But you can plug a 7 port Hub in on oneof the Ports and in to that you can pluf a 2nd 7 port Hub. If memory serves me you can daisy chain up to 127 devices.