About boosting...

[Ryuusenpai]

If i decide to purchase a SMN lv60 boost on this account on my main character, will i be able to purchase a another lv60 boost of a class on my second character on the same account?

[Valkyrie_Lenneth]

* You may only purchase one job specific Tales of Adventure item per service account. Moreover, this item can only be used on one character.

Per service account.

[Colt47]

To expand on this, the limitation is just there until patch 4.1. So if you buy a boost right now that isn't the only boost you can buy from now to eternity, just they are trying to keep people from going boost crazy and doing weird stuff.

I boosted DRK on a new character just to try out the service and see how it's like. So far I like it, but man does it feel odd running around as a level 60 character without anything unlocked yet. On the up side, the story quests are ludicrously easy.

[LunaFaye]

While it is true that this seems to be a temporary thing, there's no official statement that this restriction will be lifted, with patch 4.1
That are player speculations and we haven't heard a word from SE if these potions become unlimited with the new patch.
As for now, just like Valkyrie stated, you can use one potion per service account. Therefore chose wisely.

[Yahallo]

The limitation will at the earliest be removed with 4.1, keyword "earliest." They are monitoring the effect of boosting so it is possible that if they notice a problem, that the one per limitation will remain.

[Colt47]

The limitation will at the earliest be removed with 4.1, keyword "earliest." They are monitoring the effect of boosting so it is possible that if they notice a problem, that the one per limitation will remain.

I highly doubt they will have issues with allowing people to boost into Stormblood. Established players will definitely not like having to go through ARR again and new players will want to play with the rest of the community. That and it's free money for them.