Blades and Complexities of HA Planning:
"I have also seen customers with 2 Blade Chassis in a C7000 6 Blades in each. An firmware issue affected all switch modules simultaneously instantly isolating all blades in the same chassis. Because they were the first 6 blades built it took down all 5 Primary HA agents. The VMs powered down and never powered back up. Because of this I recommend using two chassis and limiting cluster size to 8 nodes to ensure that the 5 primary nodes will never all reside on the same chassis."
More good info at the source.