If I import an audio file, Sonar predictably creates a single track.
 
But. No matter what I try, importing a midi file creates 10 midi tracks (9 are empty, 1 contains the imported file).
 
There has to be something simple I'm missing. Thanks for your help.

you say import ? how are you importing ?
when inserting a synth, the soft synth properties window appears select "simple audio track, not multiple audio outputs.

I'm choosing File-import-import midi. Not inserting a synth, only a midi file/clip.

It is probably MIDI type 0 format file, with all the data on channel 10.

Inserting this file into Reaper and Studio One only creates one track.

It would appear different DAWs import format 0 differently assuming my guess about the file format and data is correct.

First, it is best to "OPEN" a midi file in Sonar if it is a say downloaded song with multiple instruments. Important data is lost when you import a midi file. I always open midi files unless it's just a drum part of a midi clip of a loop. But even then I never import I just drag and drop from the media browser. 
 
And if this is someone elses midi file then I beleieve if they had 10 tracks, even blank, then that's what you get.. it's what was saved and that seems to be how midi files work.  no big deal just delete them. Like scook say's it's probably a midi 0 file which is long ago forgoten from the day's of sequencer playback. 

Format 0 midi tracks have ALL data on a single track, even if it's 16 channels. A format 1 saves midi data by a different track for each channel.
 
That Sonar imports a format 0 midi file and then creates multiple tracks is, in truth, an incorrect method in my view. It should be up to the person if they want to futher split the midi file 0 into multiple parts.
 
If it IS a format 0, the Sonar should only create a single track.

jeff@mountainmirrors.com
No matter what I try, importing a midi file creates 10 midi tracks (9 are empty, 1 contains the imported file).

Does this apply to any midi file that you import or just one specific midi file?

Just checked. Yup, it's just that file so far. In that case it's not super annoying.