Electrons have no relevant mass, and protons and neutrons both have the mass of 1u. If Calcium has the number 20, it has 20 protons (and electrons). It's mass is about 40u. Core Mass - Protons = Neutrons 40u - 20u = 20u, this means Calcium has 20 electrons, 20 protons and 20 neutrons.
From the periodic table, the atomic number of calcium is 20. This is the number of protons in an atom of calcium. In a neutral atom, the numbers of protons and neutrons are the same, so in a neutral atom of calcium, there are 20 protons and 20 neutrons.