Which SCSI variant supports up to 15 devices?

Wide SCSI supports up to 15 devices on a single SCSI bus. This capability comes from its design to connect more devices simultaneously than its predecessor, Narrow SCSI, which can only support a maximum of 7 devices. The broader bus design of Wide SCSI allows for a wider data path, specifically utilizing 16 bits per transfer rather than 8 bits, which facilitates the connection of more devices within a single SCSI chain.

Ultra Wide SCSI, while it also allows for a greater number of connections, does support up to 16 devices and operates at higher speeds, meaning it is not the direct answer to the question regarding the device limit as specified in the question. Fast SCSI and Narrow SCSI, on the other hand, are even more limited in terms of the number of devices they can support.