I always buy them two at a time, because the front ones always wear out faster than the back ones - unless you are a hoon who spins your tyres of course.
People who sell tyres will tell you you should buy four and chuck away two tyres with 20k wear still in them, but i always have them put the back wheels on the front and the new ones on the back. The reason being that back tyres wear more evenly, so your new tyres will last longer if you use them on the back for a time.
The spare tyre should also be rotated - get the spare changed as one of your two new ones, and use your best old one as the new spare.
If you want to have four new tyres each time, you should rotate your tyres every 20k - just ask the guy doing your service to move the front to the back and visa versa - will take 2 minutes when they have the car on the hoist.