To create an ArcGIS Knowledge Server site, you install ArcGIS Server and authorize it with an ArcGIS Knowledge license. Therefore, the system requirements to run ArcGIS Knowledge Server are the same as the system requirements for ArcGIS Server.
The only additional requirement for ArcGIS Knowledge Server is that the site must be able to communicate with the graph store. See Ports used by ArcGIS Data Store and the port diagram available from ArcGIS Enterprise system requirements for more information.