Rules to identify the most stable resonance structures While studying resonance from this Chemistry LibreTexts article, I found the rules to follow in order to decide which structure is the most stable. Among them:


The structure with the least number of formal charges is more stable
The structure with the least separation of formal charges is more stable.


Firstly I'm not sure how to interpret the "number of formal charges" in rule 1: does it refer to the number of atoms with non-zero formal charge or to the magnitude of each formal charge? It confuses me a bit also because the total number of formal charge (the sum of them) is clearly constant (and equal to the total charge or molecule).

Secondly I don't understand which of these two rules (1 or 2) is more important and can dominate the other one.

For example two possible structures of $\ce{CNS^{-}}$ are 
$$\ce{C=N=S}\qquad\ce{C#N-S}$$

The formal charge for the first one are (in order) -2, 1, 0 while for the second one are -1, 1, -1.

The first is preferable according to rule 1 (interpreting it as the number of atoms with non-zero formal charge) while the second is preferable according to rule 2 since the separation of charge is not as strong as in the first structure (I hope I'm understanding correctly rule 2).

Am I reading incorrectly the two rules or is there one that is actually "stronger" than the other?
You will encounter situations such as these when the two rules seem to contradict each other, or they confuse you.
However, when this happens, you must use another criteria to determine the best resonance structure. Specifically, you must take electronegativity into account. 

What this means is that the most electronegative element should have the lowest formal charge, and the most electropositive element should have the highest formal charge.
This is because the electronegativity "rules" explained above are what are likely to happen (that is how thiocyanate is likely to appear). This makes sense because the higher EN and element has, the more likely it is to "hog" the electrons and have a lower formal charge.
When you use these rules to evaluate the resonance structures of the $\ce{CNS^{-}}$ ion, you can figure out why the first resonance structure is preferable.