The mass number is used to calculate the number of what in one atom of an element?
The Atomic mass is used to calculate the number of Neutrons in an atom.
Every atom is composed of Protons and Neutrons forming a tight compact nucleus orbited by electrons. The Atomic number of an element tells how many Protons the nucleus has. This is important because it determines how many electrons the atom has and consequently, its chemical properties. The Atomic mass (rounded to the nearest whole number) is the sum of the Protons and Neutrons in the elements nucleus, since their masses are nearly identical (Neutrons have one electron's worth more mass than Protons). You subtract an element's Atomic number from its Atomic mass and you get the number of neutrons the element has in the nuclei of its atoms.