Are you asking how many for the balanced venting system? Figure what you need in total vents, divide by 1/2 intake at soffits, 1/2 exhaust at ridge, compute each static vent, as per manufacturer, to find number required. Distribute them evenly. http://www.airvent.com/homeowner/pro...it-specs.shtml
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17,000 dryer fires a year, when did you last clean the inside of the dryer near motor or the exhaust ducting?