No, the atomic mass number (also known as the mass number) is not always a whole number. The atomic mass number represents the total number of protons and neutrons in an atom's nucleus. Since protons and neutrons have masses that are close to one atomic mass unit (amu) each, the atomic mass number is generally close to a whole number.
However, in reality, the mass of protons and neutrons is not exactly one amu. Instead, they have slightly different masses. Additionally, some elements have multiple isotopes, which are atoms of the same element with different numbers of neutrons. Since isotopes have different numbers of neutrons, their atomic mass numbers will vary.
As a result, the atomic mass number of an element is often a weighted average of the masses of its isotopes, taking into account the relative abundance of each isotope in a given sample. This weighted average can lead to non-whole numbers for the atomic mass number. These non-whole numbers are typically expressed with decimal places to indicate the average mass of the isotopes.