Nope you cannot be banned for this. IF they decide to then they will make a public announcement just as they did with multi boxing on wow.
I have a pretty nice rig and running 5 is touchy. It will randomly shut 1-3 of them down to to memory and heating issues.
The majority of casual gamers here probably can’t manage any more than 2 screens. My son runs a 3060 with a decent setup and 3 will run but randomly shuts the others down which is annoying.
You alt tab
It is mainly for Enchanting yourself or others while you play in same game on your main account and for using a BO barb to bo your Main.
Exactly blizz will not allow /player8 command online because so far enough people are purchasing multiple accounts
IE = they make a shizzle ton of money off botters