I think you are only able to get this information "outside of the tooltip" so far if you "really start the writ quest".
After that one could use the API function here:
Code:
* GetJournalQuestRewardInfo(*luaindex* _journalQuestIndex_, *luaindex* _rewardIndex_)
** _Returns:_ *[RewardType|#RewardType]* _type_, *string* _name_, *integer* _amount_, *textureName* _iconFile_, *bool* _meetsUsageRequirement_, *[ItemQuality|#ItemQuality]* _itemQuality_, *[RewardItemType|#RewardItemType]:nilable* _itemType_
I'm not aware of any other way to determine the amount of writ vouchers you get via e.g. bag and slotIndex or itemLink.