The element sodium has 11 protons, which is denoted by the atomic number. Because protons are electrically positive and repel each other by the electromagnetic force, sodium needs 12 neutrons in order to be stable. The residual strong nuclear force exchanged between baryons keeps the neutrons and protons together.
Sodium has at least 20 isotopes, which are versions with varying numbers of protons or neutrons. The most common version of the sodium atom, with 11 protons, is the only stable variant. This suggests that 11 protons and 12 neutrons is an especially stable configuration and represents a near-perfect atomic balance.