Does the bot use work in the background and can I run more than one bot at the same time same computer without extra work?

No, the bot does not run in the background, meaning you can not minimize the window and browse the internet while it is running.

No, you can not run 2 bots on the same computer at the same time. You can group bots on different computers though.

But only if you subscribe with more then one account right on here?

Yes, you will need 2 seperate accounts.