Hello:
Start with page 14 in the Hauptwerk User's guide (ver 4.2.1). You can also search this forum for audio interfaces for more up to date products (.i.e the Echo Audiofire is no longer available and Cymatic just introduced a 16 channel unit). If you reside in the USA, then search on-line retailers like Sweetwater or Musician's Friend for audio interfaces. Prices vary by grade (average, 'pro-sumer', & professional), features, and number of ports available. Keep in mind with Hauptwerk, its the number of audio output ports that should be of interest to you.
If you find something that appeals to you, you can search the forum and see if there are any posts regarding it.
Welcome,
Danny B.