depends. In your average computer you can accomodate 4 ide devices. Usually 1 or 2 Optical Drives, and 1 or 2 hard drives. If you put a Hard drive on the same channel as a optical drive it runs really slow. Some motherboards support up to 4-6 Sata drives so you may be able to add 4-6 drives that way. Also you could get a PCi card that gives you extra capabilites. Tell your system specs.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.