Beware, more vents does NOT mean more ventilation. (that's the point, right?) A SMALL number of LARGE "vents" should allow the most airflow, but the specific aerodynamics of the whole shape with your head inside will determine the actual ventilation properties. Don't be fooled by fancy looking designs that are more fashion than function.

There's only one way to know what ventilates best: try it on. (Alas, a luxury many of us don't have...)