While drums today can often be mixed with upwards of 16 microphones, a skilled recording engineer with an excellent player (Stevie) on a well tuned-kit in a good room can capture a fantastic drum sound with a single overhead microphone and a spot mic for the kick. You'd be astounded how minimal it can be when the ingredients in the room are correct.
That said, there's a third possibility. That they tracked the drum kit with a full compliment of 8+ mics and then recorded the blend of these mics to the three channels of tape that ended up on the final 16-track reel of tape. I have a copy of these exact stems and it's very clear to my record-engineer ears that these were indeed, just one mic per tape channel. No summing was done.
Frankly, it's not even a good drum recording by the standards of that era, but Stevie's writing and performance are so brilliant that the record is great anyway.
Also, to chime-in on whether or not Tonto is featured on this track; The only possible candidate is the Moog synth bass. But the performance is no different than what you'd get with a Minimoog. So even if they performed it using one of the Moog modules on Tonto I'd say it's not a usage that highlights anything extraordinary about what Tonto was capable of.
Why did they use so many tracks to record the same performance with different effects when they could have easily applied only the effects they liked in the final mix afterwards?
There's just no way. The drums themselves need at least 4 tracks at a bare minimum (stereo pair overhead, snare and kick), but far more likely the drums used and preserved 8 tracks for final mixing (which is relatively standard). An engineer simply would not waste 8 of 16 tracks on Clavinet. What is more likely is the 8 tracks of Clavinet were indeed actually recorded but then bounced down to two tracks (panned for stereo), freeing up the remaining 6 tracks for isolation of other instruments during mixing.
Scratch 1 track (everything channelled here initially, replaced later, so 0 track)
Clavinet 2 track (initially 8, bounced down to 2)
Drums 8 tracks (overhead pair, snare, kick, HH, tom toms, floor tom, last for one of bottom of HH or snare or outside kick)
Bass (Moog) 1 track
Trumpet 1 track
Sax 1 track
Vocals 2 track
Replacing the scratch leaves 1 track for Tonto, (but it sounds to me like the trumpet is doubled, 2 separate tracks performed the same, oppositely panned wide).
I simply don't know what to listen for to hear the Tonto.