On Nov 11th, you will need to have each wow account associated/merged with a B.Net account in order to log into the game. Prior to the 11th, there is no restriction.

It is free to merge accounts, you just need one valid email per B.Net account you create. Each B.Net account can have up to 8 wow accounts merged into it.

If you don't use the Authenticator, then one or multiple B.Net accounts is the same thing. Enter the B.Net email address (in the place of the wow account name) and then the password.

With an authenticator for increased account security, there are log in issues to be aware of for multiple wow accounts on the same B.Net account.