The number of solar attic fans you need will depend on your attic's temperature and size, along with your roof's slope. Every case is different, but generally speaking, here are the recommendations (4)
Low-sloped roofs: one solar attic fan per 1200 sq ft
Medium-sloped roofs: one solar attic fan per 800 sq ft, plus one fan per additional 400 sq ft after 2000 sq ft
High-sloped roofs: two solar attic fans up to 1200 sq ft, plus one additional fan per extra 400 square ft