Hi, I'm new to Sonar (all my experience is with Fairlight systems). I purchased X3 Studio and a Focusrite Scarlett 18i20 USB interface. When I look at the available audio inputs I only see 2 inputs from the Scarlett.
 
Is there a configuration procedure for exposing the other inputs?

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).

Thanks overkiller - I'll try this when I get home

Sonar prefers that you use ASIO drivers in preference to WDM. The wave profiler only seeks out an makes Windows type drivers available.

They are accessible via Preferences> Audio tab.

Thanks Mike, and overkiller again.
I reconnected when I got home and selected ASIO drivers - now everything has sprung to life.
The Focusrite device appears as its own item in the list of input sources, with all the channels. As someone else noted, the numbering is strange (always odd numbers, each of which gives you a left, a right, and a stereo) but easily negotiable.
All the best, Andrew

Glad its working  My US-1800 shows all the channels including left, right and stereo.  For whatever reason when adding a new track, Sonar defaults to "stereo" for the US-1800 which I have to manually change.  No big deal, just something I have to do.  You might find that there are some additional steps you need to take as well.  Once you figure out what needs to happen you will get into a smooth workflow.