I don't know exactly how many, but i can tell you that it is a lot. I did do side quests, but that is because they can give anywhere from 5,000 to 13,000 exp. i would say that it is worth doing.
If you are having problems in later worlds then I would highly suggest doing Wintertusk because it will over level you for the other worlds and you can breeze through them. Also some spell quests require you to enter areas in Wintertusk.