Focusrite Scarlett 18i20 User Manual page 16

Once the Scarlett 18i20 is set as the preferred Audio Device* in your DAW, all 18 inputs and 20 outputs
will appear in your DAW's Audio I/O preferences (note however that Ableton Live Lite is limited to a
maximum of four simultaneous mono input channels and four simultaneous mono output channels).
Depending on your DAW, you may need to enable certain inputs or outputs before use. The two
Input Config
examples below show two inputs and two outputs enabled in Ableton Live Lite's and
Output Config
pages.
* Typical name. Terminology may differ slightly between DAWs.
16