Answer :
the number is the mass number
the mass number is the number of nucleons (total number of protons and neautrons in the nucleus) so in hydrogen-3 there are 3 nucleons and as hydrogen has 1 proton, there are 2 neutrons
carbon 12 has 12 nucleons (6 protons and 6 neutrons) and carbn 13 has 13 nucleons (6 protons and 7 neutrons)
