How often you need to have x-rays depends on your oral health. A healthy adult who has not had cavities or other problems for a couple of years probably won't need x-rays at every appointment. If your dental situation is less stable and your dentist is monitoring your progress, you may require more frequent x-rays.
If you are not sure why a particular x-ray is being taken, ask your dentist. Remember that dental x-rays deliver very little radiation; they are a vital tool for your dentist to ensure that small problems don't develop into bigger ones.