Pearl Charles of the Driftwood Singers uses a 21-chord Chromaharp and only uses three chords.

http://www.youtube.com/watch?v=qMDHyaLW31o

But she is, in my own experience, the exception. Most serious autoharpists who perfrom for others, even if just at social gatherings, use 12-slot instruments and load up the chord bars they plan to use for a set. I have a 12-slot 'black box' Schmidt and over 30 chord bars for it, some of which I haven't used in years.

For a single key diatonic conversion, a 15-slot harp is nice because you can have your 9 basic chords, a Maj7, two sus4's, two M6/m7's, and an add9 or possibly a subtonic Major.

21-chord Schmidts tend to develop mechanical problems; however, making up a Bm bar and substituting it for the one you use least is not difficult.