Very similar to choosing the right size furnace and air conditioner for your home , you would the right amount of ventilation needed for your attic. Insufficient ventilation can lead to moisture problems during the winter and decreased energy efficiency during the summer but too much ventilation can be just as bad, if not worse. Roof vents create an additional roof penetration, essentially another place of vulnerability where leaks can occur. Some vents are necessary, but you don't want to needlessly increase the number of roof penetrations.
So, how much ventilation should you have? Without exception, you should talk to a professional to determine what your home requires. Generally speaking, you need a ratio of 1:300, where for every 300 square feet of ceiling space, you need 1 square foot of attic ventilation.