How would you determine the number of protons an isotope has?
Isotopes are by definition atoms of the same element with different types of neutrons.
Note that the number of protons does not alter across isotopes. If it did, you'd have an entirely different element, not an isotope of the same element.
Therefore, if you're given the element for which you're asked to find the number of protons in, just consult a periodic table, and look at the atomic number, which is typically found right above the element symbol, like shown below:
Hope that helps :)