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.
Excellent suggestion, I'm sure his engineering guys will be thinking about how to use the least space (especially if we are using a third party data centre). But if you see no reply from Ken about this think about putting it into a question for him for the update next week/PM'ing him. We should all be giving Ken our best advice right now as he needs to get the execution of our farm right first time.