Answer is: most chlorine atoms contain 18 neutrons.
Chlorine atom has atomic number 17, it means it has 17 protons.
Isotopes are chemical elements with same atomic number, but different mass number (different number of neutrons).
Isotope of chlorine-35 has mass number 35. Mass number is sum of protons and neutrons, so number of neutrons in chlorine-35 is 18 (35-17).
Isotope of chlorine-37 has mass number 37. It has 20 neutrons (37-17).
Because the average atomic mass is closer to chlorine-35, there is more this isotope in nature.
