100.
DBAN has been run on a computer with 100 SCSI hard disks. There are no artificial limits in DBAN, but PCI bus bandwidth is usually the limiting factor in practical application.
Most desktop computers will have a performance sweet-spot of four disks.
If you are building an appliance to wipe loose disks, then the most important component is a motherboard with multiple PCI buses. CPU speed and memory size are unimportant.
