When you say there are 18 tracks (channels) on the BFD mixer you are right but you won't need to output them all separately. I'm wondering if perhaps you are counting things like there are three channels for the snare, two channels for the top and one for the bottom? You don't have to route them to individual outputs, most commonly you would send them all to one output.
When I say "all", I mean the three snare channels you would rout them all to the same output. If you look at the template (link at the bottom) I uploaded on th fxpansion forum you can see I have routed the three snare channels to an axillary bus, and that bus is then routed out to sonar.
I think also, unless you have some third party effects that you can't find in BFD 2's bundled effects or you think your third party effects are better (if they are the ones that come with sonar that probably wouldn't be the case) then there is no need to use sonars mixer.
You could just having BFD routed to the master out and then learn to use BFD's very excellent mixer which I think in many ways is better than sonars. If you decide to try using BFD's mixer and just using the one master out I would still recommend using the "all synth audio outputs stereo" The reason being if you try the above method and decide you'd rather be using effects that aren't in BFD then those tracks will be there for you and you'll just have to change the outputs in BFD's mixer. You can hide all of the other 17 of sonars tracks using the "track manager"
Have you watched any of the tutorials for BFD 2? Like this one (
http://www.youtube.com/wa...P73pk&feature=related) Also, did you see I uploaded a sonar/BFD template that has a drum loop loaded and is pre-routed so you can have a look to see how it can be set up?
You can find that one here
(
http://www.fxpansion.com/...pic.php?p=72050#72050) You don't have to have it set up that way but once you get to see how it's done you can make your own and save them as templates. BFD and sonar together give you very flexible routing options but they don't have to be complicated, that's up to you.
Good luck
Steve