It depends on the size of the soffit vents (which depends on the slope of the roof and some carpentry details). You can go measure it yourself and add it up -- but make sure you are measuring the right dimensions. The correct dimension is not necessarily the area of the vent itself, but the area the air will have to go THROUGH. So, like, if you have a shallowly-sloped roof, you could have 2' wide soffits, all just screened, but to get into the coop that air might have to go through an opening just 8" high above the top plate of the wall -- that 8" dimension, times the length of the vents, is what counts. So, do all the multiplying and adding, and come up with yer total square footage of vents.
It is really hard to prescribe exactly what number will be adequate, because it depends so much on climate, coop details, # chickens, how much they're in/out, weather, management style, etc.... but it is probably smart to aim for something like *at a bare minimum* 1 sq ft per 10 sq ft of coop floor space, and really 1 sq ft per chicken is a better number because there are many situations where you'll need/want that much. Of course more than that is good, too, especially if you get hot weather.
Good luck, have fun,
Pat
