How many carbon atoms of mass 12.00 a m.u. are in 12.00 g of C?

To a first approximation, there are Avogadro's number of such carbon atoms, i.e. $6.022 \times {10}^{23}$ carbon atoms.
By definition, there are Avogadro's number of carbon atoms in $12.00$ $g$ of ""^12C. The atomic mass of $C$ on the Periodic Table appears as $12.011$ $g$ because of the existence of ""^13C and ""^14C isotopes, which slightly inflate the molar mass.