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My obvious answer is straight up, as many as possible!
But typically that means going for a full week, say from a Friday to a Sunday. Realizing that both "end days" are eaten into by travel, that leaves a solid five days for parks. A nice sweet spot for ticketing purposes, to which I'll also add Park Hopping. This way there's lots of flexibility all around. You could also drop down to three, allowing for two days to relax at the resort at your choosing.
If you can help it, it's worth pointing out that traveling on off days can make your trip cheaper! Say, fly out on a Tuesday and return on a Monday (or whatever) - as weekend days are the most expensive to travel (i.e. Friday - Sunday).