Answer:
Same number of protons.
Step-by-step explanation:
Isotopes is defined as the atoms of same element which are exist in different forms.
The posses the same number of proton but different number of neutrons. Different isotopes of same elements posses different masses. Different isotopes of the same elements posses same atomic number.
The atomic number is decided by the number of protons and atomic masses are different for these isotopes which suggest different number of neutrons.