When I bought my home the builder didn't bother to install any soffit vents. None. After learning more about attic venting I decided to add them myself. I also have two levels, and added fifteen soffit vents, evenly spaced, on each level, so a total of 30 vents. I made a cardboard template to help keep them straight and used a jig saw to cut the holes for the soffits. a well vented attic will keep your house cooler in summer and prevent ice dams during the winter.