I do plan to expand to the motherboards maximum potential at some point and I was looking at some of those 2400w server PSU. I'm still just getting my foot in the door at this point though. I likely will not be dual mining initially.
A 2400W would be able to run at least 10-11 card dual mining. The problem with the Asus B250 mining mothrboard is the PCI-E slots are divided in three groups, one of seven PCI-E slots and the other two with six. Each group has their own power supply 24-pin input, so with that motherboard it may be more efficient to use a 1200-1300W PSU for each of the PCI-E groups.