Using one drive on an external connection is bad enough, don't even consider more than one. You would want internal, either IDE or SCSI. SCSI is better for multiple drives and multi-tasking efficiently, however, almost no-one makes modern SCSI optical writers anymore. You could purchase some IDE-SCSI adaptors, ACard makes some, but they cost almost as much as the burners would themselves. I would purchase a few CMD/Silicon-Image 0680-based PCI IDE (not RAID) controller cards, since they support ATAPI devices, and then connect the burners to them, one per channel, jumpered as Master. For writers, I recommend the NEC ND3500A, they are inexpensive but good. If you only want to burn CDs, purchase several Lite-On 52x burners instead. For software, I would recommend Nero Enterprise Edition, or DiskJuggler. Not sure if the latter is still being made any more, you would have to check. I would recommend using a nice fast HD as the source drive, a WD 10K RPM Raptor would be a nice choice. Make sure that you have plenty of RAM and a decently-fast CPU, I would recommend 1GB+ RAM and a 2Ghz+ CPU.
Alternatively, if price is no object, then I would recommend a P4 Xeon-based workstation board, with 64-bit PCI and onboard 64-bit PCI SCSI, and use a whole mess of those IDE-to-SCSI adaptors. That way you wouldn't have any issues with PCI bus bandwidth, and could scale higher than about 4-6 writers, if need be.