Most ski helmets have between 8 and 14 vents, and the right number depends on when and where you ride. If you tend to run hot or do a lot of spring skiing, look for 12 or more vents to keep air moving through the helmet. If you ski in cold climates or run cold, fewer vents (6-8) will help trap warmth.
That said, the total count matters less if the helmet has adjustable vents. Most modern ski helmets let you open or close vents with a slider, so you can dial in airflow on the fly. A helmet with 12 adjustable vents can be just as warm as one with 8 fixed vents when you close them.
Also keep in mind that vent placement and internal channeling matter just as much as the raw number. A well-designed 10-vent helmet can outperform a poorly designed 14-vent one. Focus on adjustability and overall design rather than just counting holes.