The element in this diagram that has the most isotopes is mercury, Hg. The pie chart for each element show the relative abundance of each of its isotopes. Mercury's pie chart shows six isotopes. Their masses are, from most to least abundant, 202, 200, 199, 201, 198, 204 and 196. Chlorine's pie chart show only two isotopes, 35 and 37, and aresenic and americium each show one isotope.
Isotopes are atoms of the same element that have different numbers of neutrons and therefore different masses. Mercury, for example, has 80 protons. This is denoted by its atomic number. The most abundant isotope of mercury has an atomic mass of 202. The mass number is the sum of protons and neutrons so this isotope has 202-80=122 neutrons.