I just started with renoise and need some help for my setup.
I have a ultranova synthesizer and a komplete audio6 audio interace (both connectable per USB).
Which audio interface should i use in combination with my windows 10 pc (just a realtek on board soundcard) ?
I dont get your question you say you have komplete audio6 so why dont you use that?
Echoing @stoiximan, just use what you got. If later you have specific issues or questions about that interface, ask about it. People use all sorts of hardware and it’s hard (or impossible) to say what’s right or best for anyone.
Also, welcome to Renoise!