"Extra" frets and JI vs ET
Instruments- discuss specific features, luthiers, instrument problems & questions
The question of extra frets comes down to what kind of music you want to play. If you are going to play traditional tunes in a drone style, you don't need extra frets. You will get used to re-tuning your melody string to get the mode needed for each song.
If you are going to play with others, especially those who play chords, if you are going to join workshops and jams and learn songs from other people's tablature, you will want the 6.5 fret. The 6.5 fret has been standard now for several decades, and most tablature assumes you have that fret. The 13.5 fret is just the octave version of the 6.5. I would suggest getting it so that you have the same fret pattern in both octaves. However, if you are getting one of Ron's small dulcimettes, the frets may be too small up high for a usable 13.5 fret. Follow his advice on that.
If you want to play blues, jazz, ragtime, pop, rock, and other modern music, you may want a 1.5 fret (and an 8.5 as well). That is the configuration on my main dulcimers now. However, remember that we celebrate how easy the dulcimer is to play, and the more frets we add, the more complicated it gets.
And remember that you can always have frets added later on. My advice would be to get the 6.5 and 13.5 but hold off on the 1.5 and 8.5 until you know for sure what you are doing and why you would want those frets.