The decision to allow multiboxing has much to do with a person's ability to be registered to multiple accounts. If we did not allow it, you would only be able to have one World of Warcraft account, ever. That would cause a number of complications that we sometimes still see today, due to a misunderstanding of our policies, but the issue would be exacerbated greatly if we were to limit things to a single license per user.
Once the determination was made the allow a person to have multiple WoW licenses registered to a single Battle.net or even multiple licenses under the same name, how many becomes entirely irrelevant.
Two, ten or thirty, or more, the rules still apply. As long as the person registered to the account is the one in direct control of those characters, it is not against our policies to do so.
Vrakthris – Support Forum Agent 6/29/2012