@Timo Burmeister I don't believe there are any recommendations written down.
Knox itself is more-or-less just a proxy server. It uses Jetty internally and will comfortably run in 8GB of RAM on most production systems.
As Knox is multi-threaded, having more CPU threads will allow you to process more simultaneous requests through Knox. I'm not aware of any performance testing done so you'd need to experiment to find out based on your expected load how many CPU threads would work, but in general 8 CPU threads should be a good starting point.
The number of NICs needed again depends on the work-load you expect also. If you're pushing large volumes of data through Knox then obviously you'll need to think about 10GbE or multiple bonded 1GbE NICs. You should probably also have a separate NIC for the external network and the internal cluster network; unless you're using VLANs or virtual IPs on a single NIC. For most starting configurations I'd say a 1GbE NIC would be sufficient.