I have a hole in one tire of my ford ranger 4x4. How many new tires do I have to...

...buy? The tire place said 4? But Ive also heard only 2. Just curious what and why the reasoning is behind four??

If the tires have more than 10,000 on them, I would suggest replacing as a set of two, and keep them on the same axle. That way the tread wear will stay even, and not affect the handling adversely. In fact, next time tires wear out, you will only need to buy two, and will have an extra tire as well just in case.