You will only need one authenticator per B.net account.
However, with multiple accounts on the same B.Net account, it will take 30 seconds per account to log into the game.



If the accounts are each on their own B.net account, you lose all the new features like the new account management page with a drop down menu to switch account management from one account to the next.
However, being on separate B.Net accounts means you can log them all in at once, without 30 seconds between each accounts log in.
One authenticator is still all that you need.



If you want to transfer an account from one B.Net which you own, to a different B.Net which you also own, I'd recommend calling Billing.