The atomic mass of any element actually represents an atomic weight average, the sum of the atomic masses of its naturally-occuring isotopes, each multiplied by their respective abundance.
Carbon has only two naturally-occuring isotopes, Carbon-12 and Carbon-13, so both of their atomic masses will contribute to the atomic mass of carbon.
Notice that the famous #""_""^14C# isotope does not contribute to the atomic mass of carbon, since it is not naturally-occuring (it is formed in the upper atmosphere from the bombardment of Nitrogen-14 atoms with neutrons).
#""_""^12C# is listed as having a fractional abundance of 0.989 (or a percent abundance of 98.9%) and an atomic weight of 12.000000 u (u represents atomic mass units).
#""_""^13C# is listed as having a fractional abundance of 0.011 (or a percent abundance of 1.1%) and an atomic weight of 13.003355 u.
The atomic mass of carbon is then determined to be
#"atomic mass" = 0.989 * 12.000000 + 0.011 * 13.003355#
#"atomic mass" = 12.011# #"u"#
This is the value for carbon's atomic mass listed in the periodic table.
More on atomic weight average here: