What is the best way to tour each park, number of days needed?

The answer is hard to give because a lot of it depends on how you think you'll want to see the parks. What I mean is whether you think you'll want to take your time and look around or if you'll want to take a long break or a few small ones. Personally I would recommend doing all of those things because that's what I like to do, so I would recommend 2 days for Animal Kingdom and Epcot as well as Magic Kingdom. Some people, however, would rather spend all day in one park going from ride-to-ride. All of the non-Magic Kingdom parks can definitely be experienced in one day, but that doesn't leave much time for re-riding or enjoying the ambiance.

Hollywood Studios is smaller and incomplete right now -- while they build Star Wars land -- so it is hard to recommend two days there, although they do have some of the best rides in Disney World. It is possible there, however, to ride each ride twice and see most of the shows in one day.