While there is a common misconception that cold air coming into the attic can affect energy efficiency negatively, the opposite is true.
In the summers, improper ventilation can trap warm air in the attic which consequently raises the temperature of the entire house causing your HVAC system to overwork. Winter comes with its fair share of issues too. In the winter, the warm air escaping from your house towards your attic can cause the formation of snow around your eaves and damage your gutters.
A ventilation system needs to be placed in such a way that an equal amount of air comes out and an equal amount enters inside. This keeps the temperature of your house at a constant. The best way to install a ventilation system is to make vents on the ends of a roof that allow for the cold air to get in; pair it with vents on the top of the roof that through the warm air outside of the attic. Both of these vents can be complemented with fans to increase their effectiveness. Normally, you should be having one foot of ventilation for every 200 feet of ceiling space.
Though these are estimated recommendations you must contact a professional for a personalized suggestion. They will be able to help you much better by assessing the type of your property and its location. Enquire about all the tips mentioned above to get the most out of your attic.