Managing Volume/Intensity etc.

Hey, Ive been reading this site for a while now, and basically following workouts presented herein. I’ve made a few modifications, aimed towards looking great, and it’s been working well.

But I’ve allways been interested in why the set, rep and intensity levels and # of excecises are set the way they are. Reading through the articles I just get the impression that its a broad spectrum.

So I’m curious, what consitutes too much volume? Too many excercises per body part? What’s too little?