It's shady, will I get banned if I farm simultaneously on multiple account ?
I need 1 copy of the game and multiple sub on same bnet acc right ?

If by multiboxing you mean mirrored commands on multiple game instances, then yes, that is bannable. They enforce this stance very strictly.
See: https://eu.battle.net/support/en/article/24258

Yes it's allowed. Like he mentioned you can't use 3rd party software for mirrored commands though

You can /follow though

https://eu.battle.net/support/en/article/24258

This post was edited by Rusted on Apr 8 2022 12:49pm