For what you want to do, you just need to add IDE controllers which will let you put 4 on each controller. You could also go SCSI and have 7 drives connected to each SCSI controller.
For something more advanced:
Procom pretty much wrote the book on CD servers. We have a couple of 64 CD racks here.
There are a number of ways to do this depending on what you want.
If you just need like 7 drives, you can put 7 drives in an external chassis and connect them to a SCSI adapter in your PC. You have to use SCSI CD-ROMS or I think Procom has a chassis that converts SCSI to IDE so you can use cheaper IDE drives.
There are models that have 7 (or more) drives in a chassis with an integrated Ethernet card that you configure via telnet or a web browser then just connect to it from whatever workstation you want. (speed limited by your network speed of course)
The bigger models actually have a motherboard in there that does management for you, security, caching, etc.