The definition of a harp guitar has changed over the past several years, and even over the past century.  Some instruments were called harp guitars not because of their additional strings but because they simply looked like a harp or had ‘harp like’ sound qualities.  You can imagine this can become very ambiguous and difficult to narrow down, so recently  the harp guitar  has been re-defined and is what is now commonly consider the accepted definition of a harp guitar:

Harp Guitar – “A guitar, in any of its accepted forms, with any number of additional unstopped strings that can accommodate individual plucking.”

It doesn’t matter if it has 1 additional plucked string, or 21 of them, as long as there is at least 1 floating over the guitar somewhere to be plucked, it is accepted as being defined as a ‘harp guitar.’  In this video Dave defines the harp guitar and gives us examples of what it sounds like, how its tuned, and what the pickup system sounds like plugged into an amplifier.