Kleeck: It won't require 190 racks... 10.488TH solution won't use 2Full racks. The question is can you make efficient use of the space.
Ken: I'd suggest you consider 1 Head + 9 Expansion per rack and keep it to a system per rack. This would be easier to manage. It would give you 140 cards per rack or 6.3TH.
Or roughly 160 racks.
I still think that's a lot of racks for 1PH, but as I said before, if we can keep all of cases in place while upgrading units to the 28nm boards then there could be a lot of benefit to ramping up to this size, if the price is right.