How many frets are really useful?

I have twenty frets on my old McSpadden, and so many in the upper scale are never used.  Is it really necessary to place that many frets on a dulcimer?  Maybe some pros would require them, but, I think, most of us would not.  I want to build a dulcimer with minimum frets.  How many would you experienced builders/players place or need on your dulcimers?