The usual answer is to go leaf - spine rather than full mesh. In order to guarantee 2:1 oversubscription in a 7 switch fabric, you'd have to get 2 spine switches (QFX5100-24Q), and run 3 uplinks out of each leaf into each spine. The question is if you need 2:1. You may only need 4:1 or 6:1.
I don't think you can guarantee a 2:1 oversubscription in a ring given that the bandwidth of your 6 uplinks is a shared resource across multiple switches, and though there may be a way to figure out the oversubscription ratio between two given specific hosts, I think it would be very complicated, and would not be consistent oversubscription across all hosts. It would be affected by how much traffic is going east vs west, and the number of switch hops between any two given hosts.