To answer your question we need to know what you mean by "How about".
20
The mass number
mass number
mass number = protons + nuetrons
There are too many elements to list how many nuetrons each one has, but as a general rule of thumb you can subtract the atomic number (number of protons) from the molecular mass number (Which is generally the number of protons and nuetrons added, since electrons weigh very little) to obtain the number of nuetrons For example, Carbon has an atomic mass of 12. Subtract the atomic number, which is 6, and you will get the number of nuetrons, which is also 6.
Number of Neutrons = Mass number - Atomic Number
The Mass Number.
nuetrons
12,13, and 26
Isotope.
mass number = protons + nuetrons
They are called isotopes.