The audio interface does not dictate how many virtual instruments you can use within Cubase. This is dictated by the power of your computer and how much memory it has. If you are going to input the notes via a keyboard then an interface with low latency is recommended like RME. The next question is will you be doing all the mixing within Cubase? If yes then you don’t need an interface with multiple outputs.
To answer anymore you will need to put a lot more information about your requirements.
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