Up to 224 devices at the same time ;-)

[uweseimet]

SCSI2PI/PiSCSI support up to 32 LUNs per SCSI ID, just like specified by the SCSI standard. This means that for each emulated SCSI ID up to 32 drives are supported. With HDDRIVER only 8 LUNs per SCSI device can be configured with HDDRUTIL. Note that with the SCSI Driver of HDDRIVER 12 32 LUNs can be used.
All in all there can be 8 SCSI devices, with one of these devices being the TT or Falcon. (Or the ST, when a host adapter with initiator identification is used.) Since one SCSI ID is reserved by the computer (initiator), there are 7 IDs remaining for peripherals. So you can add up to 7*8 = 56 peripherals with ACSI (where only 8 LUNs per device are possible) and 7*32 = 224 peripherals with SCSI.
With real hardware it is not possible to connect that many devices, because most devices only support a single LUN, and even SCSI memory card readers do not have more than 5 LUNs, typically one per memory card slot. With SCSI2Pi/PiSCSI things are different, because the number of devices and LUNs does not depend on your hardware, but on the number of emulated devices. A setup with 224 drives is exotic, of course, but technically it is possible.
In case SCSI2Pi is used in combination with physical devices it can be useful to assign just a single SCSI ID to SCSI2Pi and assign image files to LUNs of this ID. This way all other SCSI IDs are available for the physical devices.