Finding Linux / Raspberry Pi compatible Audiointerfaces was difficult some years ago but it’s getting much better. Here are some examples of interfaces that worked or did not work for me with my test-audio-Pi:
with symmetrical inputs/outputs:
with unsymmetrical Connectors:
Conrad USB-Soundbox 7.1 (ca 35 Euro): 8 ch input or output working, full-duplex is a problem depending on the pi-workload, for I&O-operations there are better options