You can have up to 8 accounts on your Battle.net account. They can all be logged in at the same time. These can be separated, if you so choose, by contacting Blizzard support via the phone.
If you're going to attach them all to the same account you'll need 1 authenticator code per login. If they were separate you could use 1 authenticator code for as many accounts at the same time [assuming you even have an authenticator].