How many soffit vents do I need?
Your attic ventilation should be a loop from bottom to top, so that convection and draft work with the system. Soffit venting should typically be at least a third of the total area, which is why you often see alternating perforated panels in aluminum soffits. In short overhang scenarios I've used full venting.



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You should consider closing your gable vents, assuming you have adequate ridge vents. The gable vents short-circuit the loop and can result in dead zones. 

Basically it's like this: The airflow and natural thermal lifting in your attic result in outflow at the ridge. That creates lower pressure in the attic, which must be accommodated. You want it accommodated by the soffit vents, not the gable vents, so that you have a continuous flow across the entire attic area.