The atomic number is a count of the number of protons in an atom. The periodic table arranges elements in order of their atomic number. By most schemes, the 20th element is the one with 20 protons. That's calcium.
The question asks about atomic numbers. That is very odd because there is only one "atomic number" for any given element. However, isotopes of a given element differ in the number of neutrons in the nucleus. So, the element could have 20 protons and 20 neutrons. The total of these two is called the "mass number." We would call this isotope Calcium-40. Known isotopes of Calcium have mass numbers ranging from 34 to 57. But the correct atomic number for each is still 20.