For me the number is nine. I do six out in the Isle of Quel Danas, and then three in Outland (the mining, fishing, and cooking daily). I have a couple of friends who do 25 every day, and I have other friends who only do one or two. I was surprised yeterday in my article about making money through the AH that there were several of you who mentioned that doing all 25 was the norm.
So vote away and lets us know how many daily quests you do every day.