As most said, do 4, Well worth the investment.
I did these on my wife's Subaru years ago, and was totally convinced on Snows, It made her a Confident Winter Driver. Now I have 4 snow tires mounted on rims for all of our cars, Her Jetta TDI, My Passat TDI, My Plow Truck, Daughters Saab, and Sons Volvo. Wont go through another winter without them.
Keep an eye out on Craigslist, you can pick up a set of four that have been used for one season for 1/2 price of new.