Visualize a SCSI bus as a chain of up to eight devices, which has terminators at each end of the chain. It doesn't matter where on the chain you put the host (motherboard) as long as the two ends are properly terminated and nothing in the middle is terminated. And the SCSI IDs are unique.
So remembering that the motherboard is one of the devices (ID7), you can have x internal devices, and y external devices, as long as the sum of X + y <= 7.
So X could be up to 7, if y = 0.