Your risk of experiencing discomfort while riding on a warm day increases with the coverage a helmet offers. Regarding thermal comfort and reducing perspiration, airflow is crucial. Making a helmet with enough ventilation is difficult. Designers are reducing material for every vent port they create. Through advanced computer modeling and software, they can predict where material can be reduced to improve airflow without putting the wearer at an elevated risk of accident injuries. Although there isn't a set number for vents, helmets with more than 20 ports and internal channeling across the head typically allow for better airflow. When you are climbing slowly on a hot day, this is a most obvious factor.