So it's been mentioned in the forums that there are less than 30 moving parts in the drive train of a Model S. It's those moving parts that present the highest likelihood of wear and tear and eventually needing replacement. If we take the less-than-30 for the rear, then add another less-than-30 twice for the front (far less than 30 for a non-motor version), it's still not that many typically replaced parts when compared to ICE vehicles and typical dealership repair centers. Okay, multiply that times the number of variants (S, X, 3) and you've still got a reasonable number of parts to deal with. Then the seldom used parts will simply be ordered from the factory, or perhaps local warehouses.
But I don't know if I agree with the assumption that stocking of parts will come into play when Tesla makes these decisions. Still undecided on the issue, but currently leaning against it.