You mean total correct?
1 per wheel is all you need. However, 2 might look "cooler".
When we did my buddies SAS he had 1 rancho 9000, on the softest setting, then added another, 2 per wheel, also on the softest setting. He tells everyone it rides better with 2, and tells everyone dual shocks are better. However what I tried to say, and what he didnt understand is that if he had just made the 1 shock a little stiffer, it would have done the same thing. 4 is, as stated above more expensive.