depends on the scsi card/controller most newer ones support 16 devices per channel - usually with the card being one of the devices on the chain (usually id#7) so 15
older cards or lower controlers at least 8 (again card being one of them)
some raid and top end cards may have more than one channel (2, 3, whatever) so say a nice mylex 2 channel raid card - 30 drives....... etc.
i guess i want specific enough, the original question pertained to a single channel. I just found out. What i was looking for was is.....on an 8 bit bus its 8 devices, and on a 16 bit bus its 16 devices