If you have an isotope of the element carbon that has the mass number of 14, what other information would you determine the number of neutrons

Isotopes are forms of the same elements which contain equal number of protons but different number of nucleus in their nuclei, thus the differ in relative atomic mass but they have same chemical properties. The mass number is the summation of number of proton and number of neutron of an atom. If the mass number is known, then one need to know the number of proton in the atom in order to calculate for the number of neutron.

It would be