You have to do 4 tires. My Dad used to have a great story about the time he put studded snow tires on the front of a FWD car their first winter in Colorado after moving from San Diego. The first time he applied the brakes in a slippery situation, it took about a nanosecond for the back end of the car to pass the front end. He drove directly to the tire store very carefully, and had 2 more tires installed while the installer laughed at the stupid Californians invading his state. My Dad wasn't pleased, but he was grateful my Mom wasn't driving!
I just had 4 Blizzaks put on my new CT 2 months ago. It hasn't snowed since, or I could be more helpful.