If you do have some extra shingles left over, save them for patching the static vents (exhaust) needed now.Then patch after cutting/installing ridge venting (exhaust). The soffit invisivent (intake) are only 1/2 the equation, exhaust vents are needed. Even a small gable vent was acting as an exhaust due the 3 air pressures the one article explained so well because they are higher than the soffits. On your gable rake is only a soffit return, no invisivents up the gable unless not pictured.
Read the sites given, let us know the outcome, thanks.
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As some one already mentioned, you can install exhaust vents with out replacing the roof.
To meet maximum numbers and fully utilize the system you would need 50 turtle vents or gable too gable ridge vent.
But, to meet minimal requirements, or just above minimal, you can install 6 turtle vents across the backside.
You really want to consider at least going with the minimal now and upgrading later when you install a new roof, because with out at least the minimal exhaust you may end up with condensation related issues, such as dry rot and mold on the interior of the roof cavity.