take a look at the CRONUS chart of accounts and read the Financial Management training material. It will explain how to do it better than any of us. It’s a standard functionality of NAV
Yes you can create group for that in which 12000 will be parent. Mark it as a header type account.
In NAV, after you create chart of account, you can see the overview of the COA. That will basically show you which ledger comes under which group in a structured manner.