how many stakes are needed for a 30 by 40 ft house?

It depends on the square footage of your entire home. For a one-story home up to 1,200 sq ft, use 10 stakes; use 20 stakes up to 2,500 sq ft. For a two-story home up to 1,500 sq ft, use 10 stakes; use 20 stakes up to 3,000 sq ft.