The atomic mass of carbon-12 is exactly 12 u. This is true by definition: the unified atomic mass unit is defined as one-twelfth of the mass of an unbound, neutral carbon-12 atom in its ground state.
Carbon-12 contains six protons, six neutrons, and six electrons. Its mass is not simply the total of those particles because some mass is converted into nuclear binding energy, but the isotope is used as the reference standard that establishes the value 12 exactly.
Carbon-12 is the dominant natural carbon isotope, making up about 98.93% of terrestrial carbon. Carbon-13 is the other stable isotope, while carbon-14 is radioactive and is used in radiocarbon dating.
A common mix-up is confusing carbon-12's exact isotopic mass with carbon's standard atomic weight, approximately 12.011 u. That larger value is a weighted average reflecting the natural abundance of carbon-12 and carbon-13.