In "Hedge Magic: Revised Edition" it says that hedge traditions can have magic defenses based on their score in their specialty art/supernatural ability. How is this specialization determined? Is it the highest scored art? Is it just chosen by the player? Can a hedge wizard specialize in multiple arts? I would deeply appreciate clarification.
I see how the wording can be confusing. You've misread it, though. Each area of specialty may provide magical defenses. Note how in the magical defenses description on page 10 it says a hedge wizard may have several defenses that apply against a single thing being defended against. And then you can check specific hedge traditions. For example, for folk witches "each folk witch power grants her a Magic Defense" (page 38), so the folk witch gets multiple defenses if the folk witch has multiple powers. This is the same as Hermetic magi getting defenses from all their Forms, not just their highest Form.
So a Hedge tradition gets defenses for each art/ ability they know/have then?
Yes, that's right. Of course, that's for each Art/Ability the hedge wizard has from the tradition, not just every one within the tradition. Un-Gifted hedge wizards will tend to only have a smaller selection of those Arts/Abilities.