Hello!
For what the numbers 12, 14, and 13 represent is the mass number, this is the total protons and neutrons
So for the first isotope since its mass number is 12, it has 12 protons and neutrons.
To find the amount of neutrons you subtract the amount of protons, 6 or the atomic mass from the mass number.
12 - 6 = 6. So you have 6 neutrons in this isotope.
For the second isotope it has 13 protons and neutrons, 6 being protons and 7 being neutrons.
Finally, for the last isotope it has 14 neutrons and protons, or 6 protons and 8 neutrons.