Most carmakers design their suspension systems so that the struts can be replaced individually. There's no need to replace all four at once unless one is leaking or otherwise damaged.
If you're only replacing one strut, it's important to have the vehicle aligned afterwards. Replacing two struts on the same side of the vehicle will also require an alignment.
Keep in mind that if your struts are worn out, chances are good that other suspension components are close to failure as well. It's not uncommon for control arms, ball joints and tie rod ends to need replacement along with the struts.