First Question Response) The way I was able to figure it out with some skills was by watching he demonstration videos that show cased specific skills, and counting how many times the damage number animation popped up on the screen.
However, there are some skills (from what I’ve noticed) that are capable of hitting multiple times in one blow; which I was not aware even from the clips. One move being Cartar Stroke.
It’s likely that there are some skills where there wont be a definitive way of knowing until you put a point (or a couple) into a skill, and then test it out. That, or by looking up specific skills using Google and seeing what information you can dig up.
Second Question Response) As for buffs, that too most likely relies on personal testing. Example: Fencers Preparation skill states that “The next [Pierce] attack you make will deal twice as much damage”. However, after reading up on Fencer I later learned that the buff from that attack only applies to the first initial strike from a multi-hit attack, and does not apply to the other multiple hits following the initial strike of the multi-hit [Pierce] attack chosen.