Can I mount a Carl Zeiss CONTAX 85mm f1.4 lens on a Nikon DF?

Would I need an adapter ring?

Not without an adapter… And although it's a great piece of glass… It isn't worth the hassle, your camera won't meter with the lens.

Fantastic lens but won't fit your camera without an adapter (if they make one).

It's an excellent lens. If it has an M42 thread, as almost all the Contax lenses do, then the adapters are freely available (I use an old Zeiss Tessar with an adapter on a Canon camera). The point is that one has to revert to the era of manual focussing and aperture setting, so photographing moving subjects can be difficult. Set the camera either on manual or aperture priority, focus with full open aperture, stop down to your chosen aperture and take the photograph.