An isotope is a form of an element where the number of protons, or the atomic number is the same but as the number of neutrons differs, the atomic mass is different. It is not necessary for all elements to have isotopes though most usually do have natural isotopes or isotopes which have been artificially created. Isotopes can be stable or unstable depending on whether they decay or not. Some isotopes have a half life which is in billions of years and can be taken to be stable.
The element with the highest number of naturally occurring isotopes is Tin which has 10 isotopes.