Pairs, but it depends on how the bulb burned out, or what caused it. Generally, when replacing halogen bulbs, I also pull out the unburnt one and look at the filament to see how much it has worn. If it has worn significantly, then pairs it is. But, if it is relatively new (less than 100hrs), and the filament looks fine, then just replace the one that burnt out. Bulbs are an inexpensive maintenence and safety item on the car.
This is completely different if the bulbs are gas discharge; I would always replace in pairs on gas discharge because of the color shift that happens in gas discharge bulbs, and that with use, gas discharge bulbs do have decrease in output.