the number of strands required in a string depends on material, the properties of that material and what you want in terms of performance. take fast flight for example. 20 or even 22 strands will give a string which will be slower but more stable and forgiving of a poor release which is also more kind to your equipment as it will absorb more of the initial shock of the shot;
These will require a larger size pin nock.
Whereas 18/16 strands will give a faster shot due to less mass in the string but will be less stable and not so forgiving on either your shot or your bow limbs as there is not as much shock absorbancy in the thinner string.
These will require a smaller size pin nock.
Dacron is the prefered choice for the wooden type trainer bows which most archers will learn to shoot with. With around only 12-14 strands. this rightly suggests that this is a much thicker and therefore heavier material than fast flight or any of the other materials mentioned here so does not require the same number of strands. This material does not lend itself to distance shooting as there is around 2-3% stretch on each shot which is like shooting with grannies knicker elastic! But is very forgiving on both shots and bows and is cheap!
If distance is your goal then thinner strings are the way to go or use a lighter material such as BCY's 8125, 452 (or 452+ for
compound) or the new GIGA material(s) but be prepared for the less forgiving nature of faster strings especialy with GIGA as there is very little stretch. (I know, I use an 18 strand GIGA outdoors)
These strings will punish a poor release but give great arrow speed and distance.