Maximum number of drives per bus type
Bus Type Drives
Narrow SCSI 7*
Wide SCSI 15*
Fiber Channel AL 126*
Fiber Channel fabric 16M*
IDE/ATA/EIDE/UDMA 2
Number of busses per host
SCSI Unlimited
IDE 2**

*Remember with scsi the Host controller counts as a device so while Narrow SCSI supports 8 devices it really only supports 7 because of this.

**Newer system have found a way to have up to 4 ide busses but only the most current os's will be able to use anything above 2 (which is beyond the scope of this site).