As others have said, get an Authenticator.

You can go with the Key Fob version, or an application on most Smart Phones.

Then the choice is one Battle.Net email with all accounts on the same email. This gets you easier account management (via drop down boxes to switch accounts) from a single log in, and a single authenticator code. It also means, you need to log your accounts in one-after-the-other with a separate authenticator code for each.

Alternatively, go with five Battle.Net accounts, with one Warcraft account per Battle Net. Then link the same authenticator to each Battle.Net/Warcraft account. This means you will need to log in five times, for account management changes, but that you can simultaneously log into Warcraft with all of your accounts at once, on a single Authenticator code.

I went with the second option, since I log into the game more often then change payment details on my accounts; I'd rather have the faster log in, then the faster game management.