A mole is the amount of a substance that contains as many particles as the number of atoms in 12 grams of what isotope?

The ""^12C isotope..................
By definition, $\text{Avogadro's number}$, often represented as ${N}_{A}$, 6.022140857(74)xx10^23 *mol^(−1) is the number of carbon atoms present in $12 \cdot g$ of ""^12C.