Composition of Nucleus

In the symbol $^{A}_{Z}X$:
Z = atomic number = number of protons in the nucleus.
A = mass number = total number of nucleons (protons + neutrons).
So the number of neutrons $N = A - Z$.
Hence the nucleus has Z protons and $(A - Z)$ neutrons.

Mass number A = number of protons + number of neutrons = Z + N; atomic number Z = number of protons.
Since $N \geq 0$, we always have $A \geq Z$; A can never be less than Z.
The equality A = Z holds when there are no neutrons. This happens only for the ordinary hydrogen nucleus $^{1}_{1}H$ (a single proton). For all other nuclei A is greater than Z.
So the mass number is sometimes equal to its atomic number.