You are correct in your understanding of atomic mass. The atomic mass of an element refers to the weighted average mass of the atoms in a naturally occurring sample of that element, taking into account the relative abundance of each isotope.
Isotopes are variants of an element that have the same number of protons but different numbers of neutrons in their nuclei. Since isotopes have different masses, the atomic mass of an element cannot be simply the mass of a single atom. Instead, it is calculated by considering the masses of all the isotopes of that element and their respective abundances in nature.
The atomic mass is commonly expressed in atomic mass units (AMU) or unified atomic mass units (u), where 1 atomic mass unit is defined as 1/12th the mass of a carbon-12 atom. The value of the atomic mass of an element is usually listed on the periodic table.
To calculate the atomic mass, you multiply the mass of each isotope by its abundance (as a decimal), summing up these products for all the isotopes. The resulting value represents the average mass of the element's atoms. This average accounts for the presence of different isotopes and reflects the typical mass of an atom in a naturally occurring sample of the element.