The ventilation will depend on where the insulation is and how much you have. The easiest may be two gable end vents, one on each side. Your roof slope may be too low for a ridge vent and a ridge vent would require soffit vents which I cant tell that you have. Whirly birds will require soffit vents too.
Remember, fresh air in, stale air out. You need a path for the air.
Whats your goal? If you are re-roofing or remodeling inside it might be best to do a no vent required insulation upgrade. There are spray foam contractors that can completely fill the space in such a way that no venting is needed. They would need access from below or above to do their jobs and all old insulation would need to be removed.
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