Carbon exists as two major isotopes, 12C, and 13C (14C exists and has a half life of 5730 y, 10C and 11C also exist; their half lives are 19.45 min and 20.3 days respectively). Each carbon atom has the same number of protons and electrons, 6. 12C has 6 neutrons, 13C has 7 neutrons, and 14C has 8 neutrons and so on. Since there are a variety of carbon isotopes we must specify which C atom defines the scale.
On the periodic table the mass of carbon is reported as 12.01 amu. This is the average atomic mass of carbon. No single carbon atom has a mass of 12.01 amu, but in a handful of C atoms the average mass of the carbon atoms is 12.01 amu.
The natural abundances of 14C, 10C and 11C are so low that most mass spectrometers cannot detect the effect these isotopes have on the average mass. 14C dating is accomplished by measuring the radioactivity of a sample, not by actually counting the number of 14C atoms.
(Yes, the sig figs are correct.)
To the correct number of significant figures
The point here? As long as the number of atoms remains the same the relative mass does not change.