The number of neutrons is 12. Typically we just round the atomic mass to the nearest whole number and subtract the atomic number from it to get the number of neutrons. Because electrons are so small, their mass is negligible, so the mass of the sodium atom is just protons plus neutrons.
In high school chemistry, we determine the mass of 1 proton to be 1 and the mass of 1 neutron to be 1. The atomic number is 11, which is also the number of protons. Simply, 23-11=12 neutrons.
