Cisco got different kind of Nexus 7k devices.
Why a Cisco Nexus 7702 with only one supervisor and the possibility for 1 blade ?
In a good Data Center design there are always redundant pairs of devices. So for instance 2x Nexus 7710 as a redundant core. But what if I do have a small network with Nexus 5600 in the core ? Then I don’t need the Nexus 7700. That is correct and only the Nexus 7K got the capability of Data Center Interconnect (DCI)
So for instance I got two DCs and I want to connect them over a L3. Here is where the Nexus 7702 comes in place. It got all the Nexus 7K features. Not only OTV, but also LISP, Intelligent Traffic Director (ITD), Remote Integrated Service Engine (RISE), VXLAN, eVPN, VPLS, MPLS.
It is also low on power. Very convenient for companies who are green. The chassis is capable to upgrade with a new blade for faster port speed.
The Nexus 7702 support a blade with 10/40 and even 100 Gb/s port speed. Due to the fact that you have 2xNexus 7702 in your design, you can ask yourself, why do I need two supervisors. It’s already redundant.
Sources : Cisco Nexus 7702 page
If you want to see all Cisco Nexus devices, use the link : www.cisco.com/go/nexus