My answer will be, it depends.
First off, which system you have. M-1 versus C-5, for instance, and that will give you a different number of coils.
I think, to some degree, you want to use common sense.
If you are treating an area close to the surface, you don’t need to stack coils, for instance, but if you are doing deep brain or hips or if you are obese, then actually reaching the area that you are trying to reach may mean to stack coils.
When I was treating my injured ankle, I used a higher power than normal and put a coil on either side of the ankle and mostly used it for a long period of time.
When I use it for sleep, I tend to need the quietest, lowest power possible and I honestly listened to the sounds of each setting to find gentler settings and I tend to put it side by side behind my head so that it doesn’t wake me up when I move around.
When I was trying to reach deep brain areas, I used the C5 and double-double stacked them. Yes, sitting in a reclining chair tucking the stacks into a big hat. I don’t do that for anything other than the deepest brain levels and hips. And, I will add that I am not an expert. It did help tremendously, but I will say that it may be even more important to not get into one pattern. The body habituates and varying things seems like the way to go, in my opinion. Taking time off for a few weeks after using a program for a while is one way that seems to help me.
I use many of the settings and intentionally mix things up and intentionally rest after a few weeks of intensive usage.