How many snow tires should I buy?
As many as you have wheels on your car. For most people, this is four. If you put only two snow tire on your car, one end of the car will have better traction than the other. This applies both on dry and snow covered roads. In the snow, the snow tires will have better traction. In the dry, the regular tires will have better traction.
The difference in traction means that when you brake, turn, or accelerate, the two ends of the car will want to behave differently. If the back end has worse traction, you will spin. If the front end has worse traction, you will keep going straight, even while trying to brake. Neither is really a nice thing to have happen in a panic situation.
But, I used to put two snow tires on the back of my....
Okay. It really is your choice. You will find that if you invest in high performance snow and pavement tires, the miss-match is a lot worse then you ever got with the $20 special bias-plies. Many tire manufacturers and retailers are now unwilling to sell just two snow tires because they do not want to be liable for the consequences.
If you do decide to only go with two snow tires, put them on the back of the car, regardless if it is FWD, RWD, 4WD or AWD. It is a lot easier to deal with the car understeering (wanting to go straight) than it is with the car wanting to oversteer (spin).
Another thing to watch out for is mixing different brands of snow tires. I know of one person who ran two different brands of snow tires on his car, and depending on which end of the car he put which brand made the car either very easy to drive or completely undrivable.
If you do nothing else, always match tires on the same axle. They should be the same brand, type, pressure, and have the same wear. (I am talking no gross differences in wear, and ignoring tire pressure changes for racing, etc.) A tire with little tread will not do as well in the snow as one with most of its tread.