Na-24 and Na-23 have the same number of atoms=11, different neutrons(13 and 12),different mass number(24 and 23)
Further explanation
The elements in nature have several types of isotopes
Isotopes have the same number of protons and having a different number of neutrons.
So Isotopes are elements that have the same Atomic Number (Proton)
number of atoms = 11
mass number = 23
number of neutrons=23-11=12
number of atoms = 11
mass number = 24
number of neutrons=24-11=13