Isotopes are atoms of the same element (which means they have the same number of protons) with different number of neutrons. For example, carbon-12 and carbon-14 are both the same element carbon, because both of them have the same number of protons; they are different isotopes of carbon because they have different number of neutrons. Isotopes are written in this format: AXZ. A is in subscript, and Z is in superscript. Somtimes in some books both A and Z are on the same sides, but still A is in subscript, and Z is in superscript. I prefer writing A and Z on different side because that is easier to type. So here A represents atomic number (or proton number, because atomic number = proton number), while Z is the mass number, which equals the sum of the number of protons and neutrons. And X represents the element using those chemical symbols in the preriodic table.Carbon-14 can then be written as 8C14 , and Carbon-12 can be written as 6C12. So how many isotopes does each element have? Well, each element has different number of isotopes, but it can't have infinite ammount of isotopes, because atom will have a limited number of neutrons. The number of neutrons of an atom really depends. Light elements tend to have about as many neutrons as protons while heavy elements apparently need more neutrons than protons. This way, atoms will have enough neutrons to interact with the protons to make the nuclei stable. Atoms with too many or too few neutrons will be unstable and become radioactive. Radioactive atoms tend to go through a series of decay and eventually the nuclei will become more stable.