8 would be the max, and you could always add another Promise ATA/100 Controller for 4 more drives (I understand they will all share one IRQ to, way cool). The limiting factor in this case though, may be your powersupply, if you plan on running 8 devices all at once, get at least a 300watter (and a good one, not just one that says 300), if you've got that and heavy cooling, consider 350 or 400.
Jay