The code requires a certain amount of attic ventilation.
Attic Ventilation shall be “cross ventilation “ and shall be 1/150th the area that is to be vented. (See ICC R806 Vents.)
However, there is one exception: The attic ventilation can be reduced to 1/300th the area to be vented provided one of the following items are met:
1) a Class I or II vapor retarder is installed on the “warm-in-winter” side of the ceiling, or
2) not less than 40% and not more than 50% of the ventilation is located in the upper portion of the attic. The upper portion is defined as: “Upper ventilation shall not be lower than 3’ below the ridge or highest point of the roof. “ (See ICC R806.1.2.)
Also, the Code says you shall maintain a 1” clearance between attic insulation and framing.
So, you can see there is clearly a benefit to having ridge vents, gable vents, etc. , but in a proportion that allows COMPLETE CROSS-VENTILATION to the attic.
Soffit vents with ridge vents obviously meet that requirement best, but gable vents and “turtle vents” (as you call them) are also effective.
So, if you have a 2,000 square foot house, you’ll need 13.33 square feet of vents if about half is NOT located in the upper portion of your attic,
You’ll need 6.66 square feet of vents if between 40% and 50% of the vents are in the upper portion of your attic.
You can easily calculate the amount of ventilation on your roof, if you use about 2” wide per foot for ridge vents (about 0.15 square feet per linear foot) and about 1/3 square foot for each roof vent. (The free area of vents are reduced due to insect screen, which is required in every vent. However, this rule of thumb will give you a good idea of how much ventilation you have.)