I finally went through every episode that came after "The Empath" in production order, and have confirmed that nearly every single one (14 out of 16) has at least a bit of George Duning's "Empath" score. The two exceptions are "Plato's Stepchildren" (which was the last episode to have original music written for it, by Courage), and "Wink of an Eye" (which does, however, use a fragment of Duning's "Is There in Truth No Beauty?" music, which was performed and recorded on the same day as the music for "The Empath" and has some points of similarity). As noted elsewhere, three episodes that feature "Empath" music were broadcast before "The Empath" itself was ("Day of the Dove," "For the World Is Hollow," and "Tholian Web"). Perhaps there are original episode scores that approach or even equal this total - 14 plus one, "The Empath" itself. I'd guess Steiner's "Spock's Brain" score and Courage's "Enterprise Incident" score would be candidates.