Mandatory Authenticators?

[MiRai]

Is a separate authenticator needed for each account?

No the same authenticator can be used on multiple accounts.

Keep in mind though you can only use one authenticator code per battle.net account at a time. You must wait for the authenticator to produce another code [like 30 seconds?] before you can login the next account on that BNet account. I called Blizzard 2 days ago to have them separate my BNet account so I can once again use 1 code to login 5 accounts. The process was very painless [minus the hour that I was on hold] and the guy I was talking to didn't even know that they had changed it to 1 code per account [it used to not be like this]. You can only separate your BNet account by calling phone support. It's always been like this and I asked the representative to confirm when I called and he said that calling is still the only way to separate them.