Question

What is it called when an atom has the same number of protons but a different number of neutrons?

Answer 1

It is called an isotope.

Answer 2

Isotopes are different forms of the same element which have the same number if protons but a different number of neutrons.
Eg: Carbon-12 has 6 protons, 6 electrons and 6 neutrons whereas Carbon-13 has 6 protons, 6 electrons and 7 neutrons
Hope this helps :)