Not sure if you'll get a direct answer from ZOS, it's possible one of the community team might see and be able to look for you though.
I believe it's per character for the limit on daily quests. So if you want to farm the next event to the fullest it will be 40 quests per character per day (if you're crazy enough that's 600 quests a day)
The fastest ones are daily crafting writs, if you're on PC you can get "Lazy Writ Crafter" add-on which cuts down on the time these take even more.
Otherwise I am afraid I don't have much advice on which quests are going to be the quickest.