I think it might depend on how
quickly six devices get activated, too. I've activated way more than six devices/apps over a period of a few years, and have never received a failure (or max auths reached) message of any kind. I believe available authorizations recycle over longer periods of non-use.
So unless you actually
plan on using more than six devices simultaneously (or try to authorize 7+ in a short time), I don't think one has to worry about hitting the authorization max with typical device/app rollover.
Mileage will probably vary, though.