Not seeing multiple inputs from Scarlett 18i20

I have a TASCAM US-1800.  I have Sonar X2 working with all the inputs (16) for the US-1800.  What I did was install the TASCAM drivers according to the TASCAM instructions.  Then when I installed and started Sonar I ran the Wave Profiler and it recognized my inputs.  Under preferences I can see the inputs.  I also made sure I used the WDM driver model, which was the only one that worked with my US-1800 in the configuration I had.  Most people use the ASIO drivers, which is listed in the same preferences area as the WDM driver.
 
For your setup I'd do basically the same thing.  I'd install the Scarlett drivers for windows according to how they're supposed to be installed (ie:  install driver, plug in Scarlett via USB, etc).  Make sure you reboot windows and turn on your Scarlett device.  Then install or run Sonar and open the preferences (hit "P").  Check the driver model (mine is set to WDM, you should set it to whatever your Scarlett recommends, probably ASIO).  Then run the wave profilier and let it detect your Scarlett and it's recording / playback abilities (bit depth and sample rate). 
 
After that I would think your Scarlett will work just fine.  When I add a track, I have to manually select the "input" and "output" for the track (selecting the US-1800 channel I want to record from).  You probably have to do the same thing (manually select a channel from the Scarlett when you add a track).