Before getting to the mounting part however, its imperious that you check if your vehicle has enough tire-fender clearance to allow the mounting of tire chains. Generally, an inch all around the wheel will be enough, given standard tire size. Larger tires or aggressive threaded ones may make things difficult for you.
Chains come in a variety of sizes an designs so make sure you get the right-sized ones and try fitting them around your car's wheels right as soon as you purchase them. Doing this later, let's say on a remote ice covered back-country path on a blizzard, and seeing they don't actually fit is not quite the award-wining scenario. All chains provide the same grip and stability except maybe the pinned and double-linked ones. The latter have the same ladder-construction as the cable type only that the number of cross links is twice as larger.
If for some reason, getting a pair or two (preferably two) of tire chains isn't a top priority, the following may kick it to the top of your shopping list: they enhance lateral stability, traction, acceleration and negative acceleration (braking). In translation, they help keep the car where it should be at all times: on track.
Snow chains and winter tires make a perfect couple so once you begin noticing snow deposits on your front alley, it's high time you leave summer behind and switch to arctic mode. When mounting your chains, unfold them and place them parallel in the front of the car to match the wheel's trails. Check for lever locks to always face the exterior, get behind the wheel and slowly drive over them. You should leave enough of the chains to pull them across the top of the wheels. Once you have done so, look for the latch in the back-of-the tire chain section, secure it and then close the lever lock to the front. Pull the chain to see if it's secure and check for proper positioning.
As simple as it may seem, it will take you a little time before getting into it. If you find mounting your winter chains difficult, take your car over to a specialized centre or try some easy-mounting chains. Such types usually come as part of a lager kit comprising detailed instructions and supplementary tools for making the job a lot easier.
Depending on the type of vehicle you own, chains can be mounted in a variety of configurations, each with distinct strengths and weak. Bottom line is, no matter the drive type (4WD, two-wheel front or rear drive) chains can be mounted on both or one pair of wheels, at your choice.
Full-time four-wheel drive cars equipped with chains all-around are the safest to drive in winter as acceleration, braking, steering and grip are enhanced to the fullest. Front axle mounted ones do about the same except one thing: securing the car's rear. Fishtailing is never fun on icy roads unless you really know what you're doing so keep a low, steady speed and you should be fine. Rear axle chain-mounted 4X4's handle and brake poorly but have good acceleration and reduce the risk of fishtailing.
As for two wheel drive vehicles, chain mounting is strongly linked to the type of drive. While front axle mounted chains on front-wheel drive cars are your best friend, rear-fitting them is a choice even worse than animal-imprint silk underwear. You will eventually find out that the car has very poor acceleration, handling and braking so why bother experimenting?
At the opposite pole, rear-drive cars are even harder to please. No matter the front or rear axle chain placement, the vehicle will still object by fishtailing in the first case and handling/braking/rear-end running in the second.
In conclusion, stick to the figure “4”. Get two set of chains and pretty soon you'll be singing a winter driver's version of “I got 5 on it”. However, don't get to peppy, there's nothing gangsta about tire chains, at least not that we know of.