I use the 8x16 vents. I have a template that I screw to the soffit with two screws. It predrills two holes and once up there I draw the square to be cut. Then pull he template off and sawsall the hole or use a light cordless circuit saw. Doesn't need to be pretty just a hole.
As for spacing that depends. Ideally you want about the same sqft of vent at the soffit as at the ridge. It's super easy to put too much in the soffits but I haven't seen a detriment to doing so. I put 52 around my parents house which is around 2 k with a two car garage.
I also prefer to over do it on the soffits because they will fill the screens with pollen and dirt and plug up over time. Also loose insulation can make its way on top of them.
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