The amount of ppl doing it is obviously depending on how easy it is to do it.
Imagine if you could play multi account on the same laptop. For sure way more people would be doing it.
Now imagine you need many devices, AND risk punishment by doing it. Many people wouldn't consider the effort. There would still be people doing it, but, by making it less accessible/more risky, you reduce drastically (well, it depends on the measures you take), to make it discrete enought, to be uneffective affecting the other players gameplay.
I'm not inventing it, just check other MMOs securities (and btw, check how they vary depending on the staff vision of the thing).