Ok, you get you need them 😊 But how many roof vents does one home really need? If your roof has a vapor barrier or is more than 300 sq. Feet, homeowners should have one square foot per roof vent. For every 150 square feet of space, you will need one square foot (or 1:150) of roof vent 😎 Your roof vents must be evenly divided with one-half dedicated to air intake, and the other half for exhaust 😉 A 2,400-square foot home would be considered a 2.400 sq. Ft. Home with a roof that has a moisture barrier would need 8 feet roof vent 
Let’s go through an example… For a home with 2000 square feet of attic floor space, you’ll first divide 2000 by 300 (2000 / 300 = 6.66). There are 6.66 square feet worth of attic ventilation. You want to create an even system so divide the area by 2, so half of your ventilation will be intake and half will be exhaust. So 6.66 divided with 2 equals 3.33 square footage of attic ventilation to provide intake and 3.33 for exhaust. You will have to convert the square footage required into square inches because vents come in square inch ratings. You do this by multiplying the recommended square footage by 144. So, 3.33 x 144 equals 480 square feet of attic ventilation. Eilene Ryder deserves a special thanks for her amazing insight. 
Gaf.com Then, it explains what attic ventilation is. How to ventilate your attic. Includes an overview about ventilation, and installation instructions. Although attic ventilation can be overlooked and not properly ventilated, it is often very important. Attics can result in increased energy costs Roof structure. GAF is dedicated to improving roofing methods and achieving better results. Visit our YouTube channel to see more information about roofing techniques, products overviews, and guides. To receive the best and most up-to-date content, subscribe today. This will help you to complete your roofing project like a professional. Learn more here: https://www.gaf.com/en-us/for-professionals/tools/ventilation-calculator (edited by Adriana Sanders on September 16, 2020) 
Bertroofing.com Also, the photo shows all three main types of roof vents. You can have problems if you put different types in an attic. You can find the following: ridge vent Both the turbine vent and convection heat rise are passive devices that rely on hot air rising. The power vent draws air from the attic’s top. If a power vent is placed next to a passive ventilation, it may draw air from the passive vent. As you can see, the power vent pulls air from both the turbine vent and the ridge ventilation. This is basically what’s happening here. System is drawing air from one part From one end of the attic roof to the other. This attic has little or no ventilation despite having double its roof vents. (We thank Jabez Rodriguez for the recommendation).