How many strings does a folk harp have?   Of course there is no definitive answer.  But there is an
answer to the question of how many strings the
ideal harp has.  It is one more string than the are on the
harp you are now playing.  There is something about the harp aficionado that as a matter of course
more as better.  

came  about from exactly the opposite direction.   The view of the harp as a folk
instrument favors fewer strings and the corresponding more cleverness and creativity in arrangement.  
The original                      had 17 strings from D above middle C imitating the range of a D whistle.  
The current harps with 22 and 26 strings are concessions to that ideal.
Seventeen strings?  What can you arrange on "just" seventeen strings?    Listen to this fellow's arrangement on the 17 strings of a folk
instrument.  When seen from the paradigm of folk music, 17 strings is a wide range.  In the case of the folk harp the soundbox big enough
for those 17 strings beginning at D is not large enough to really mellow out a lot of the tinniness of the sound, so the size of the box is
increased a bit and, hey, why not fill it with strings?  So the thinking behind these harps is not that they are a diminished and compromised
version of a 4 or 5 octave harp but rather that they are more like and expanded [and thus compromised] version of an already self-sufficient
folk instrument of an even more modest range.
Arranging for and playing within the 2½ to 3 octave range of a folk instrument takes skill and insight.  Alas with a harp of extended range
what often happens is that frills, runs, gratuitous arpeggios, a thumping bass line are often substituted for that insight and the results are
then a muddle of sounds in which one can scarcely make out the tune, a phenomenon I call                     .
The design and construction of the small harp is not simpler nor easier because it is small.  In fact it is just the opposite.   One might
imagine that the top 3 or 3½ octaves of a large harp could be cut off, a new pillar installed, and this would be as good a small harp as is
to be hoped for.  But the fact there are many construction methods and conventions used in building large harps that the builder cannot
"get away" with on a small harp.  Using the same construction techniques that work well on a large harp in building a small harp  
results in decreased volume, flat tone, and a decided plinkiness to the sound.  As the size of the harp settles into the ideal small size, the
harp must employ every small advantage to increase the quality of the sound.
The length of the strings determines the size and shape of the harp.  Not the other way around.  Above all else it is the
length  which determines the characteristic sound of a string.  The harp builder will have a particular quality of sound in mind
for a harp and there will be only one length of string that results in this targeted sound.  A harp cannot be made to fanciful
dimensions and then the strings coerced to fit it.  

The harp balances the flexibility and mass of the soundboard to the mass of the neck and pillar.  When the sting is
played, the maximum energy is dissipated by the soundboard and soundbox when the other end of the string encounters just
the right mass to reinforce the vibration and neither dampen it by being to heavy nor dissipate it by being to light.  t is not the
very lightest harp possible that is the best but rather the harp that is of the optimum mass.

The strings are not obstructed with any devices but rather left to vibrate freely from tuning pin to shoe.   Bridge
pins and sharping levers or blades diminish the sound of the string enough that in the more sensitive small harp it is noticeable.

The tuning pins are located as low on the neck as possible.  The tuning pins high on the neck exert a far greater torque
on the neck than those mounted at the bottom edge of the neck.  For the same reason that this compromises the structural
integrity  of the harp, it also uses part of the string's energy dealing with this rotational force.  For the same reasons the
string must placed as close to the wood of the neck as is practical.

The back of the harp is permanently closed.  With the back open on a large harp for access to the strings, the harp is
vibrating like a banjo or drum.  This type of vibration on the great surface area of the soundboard of a large harp results in
quite a good sound.  But as the harp gets smaller it begins to sound more like a banjo with a decided plinky sound to the
strings.  When the back is closed, the small harp shapes the sound more as a violin does.

The material of the soundboard is critical.  The large nylon harp with a spruce or similar soundboard has enough mass
and size that little damping of the complex components of the string vibration occurs.  But for the small nylon harp spruce
soundboards can sound flat. A better choice is hardwood laminated in many thin layers so it can be very thin and flexible and
yet retain enough mass to match the tension of the strings.   For wire harps solid hardwood with a rather good deal of mass
is needed to match the greater tension of the brass or bronze strings.