Two types of SCSI devices are accessible via the sd driver:
"direct access" devices which are usually magnetic disks. [SCSI peripheral device code is 0]
"Optical memory devices" which are often called MOD disks. [SCSI peripheral device code is 7]
The sd driver is capable of recognizing 128 disks when it is loaded at kernel boot time or later as a module. However, once it is loaded, it will only recognize a fixed number of additional disks. The number of additional disks that can be accommodated is set by the kernel configuration parameter CONFIG_SD_EXTRA_DEVS whose default value is 40.