I am able to pin 7 VCPUs in my Windows 8.1 VM on 6.0.1. In the VM, task manager shows 7 virtual processors. My processor is an Intel Xeon E5-2620v3 - 6 cores (12 w/ hyperthreading).
I wonder if your issue is related to pinning VCPUs from separate processors . I would be curious to see what would happen if you tried pinning 3 VCPUs from each processor (i.e. cores 3,4,5,6,7,.