To find out how many flood vents you need to install on your home you will need your Elevation Certificate because it explains the square footage of your crawlspace and garage. Also, it tells what flood zone you are located in.
Then, you will need to decide on which type and model Engineered Flood Vent you want to install. Once, you have decided on a flood vent to install in your home or garage. You will divide the square footage the Flood Vent is "Rated for" into the square footage of area being covered.
Example: ICC Breakaway Flood Vent - Model# CSBA816 covers 305 sq. ft. and the crawlspace is 1,200 sq. ft. So, 1200 / 305 = 3.93 meanig you need 4 each flood vents to cover that area.