The mass number of a particular isotope must be determined experimentally using mass spectrometry.
The mass spectrum of naturally-occurring neon shows that neon has three isotopes: Ne-20, Ne-21, Ne-22.
The number of neutrons for a particular isotope can be determined if the mass number is known.
Number of neutrons = mass number - atomic number (number of protons)
For neon (atomic number 10)
Ne-20 has 10 neutrons.
Ne-21 has 11 neutrons.
Ne-22 has 12 neutrons.
Ne-20 has 10 neutrons.
Ne-21 has 11 neutrons.
Ne-22 has 12 neutrons.