There are three atoms of Sn (Stannous or Tin) in 356.13 g of Sn.
One atom of Sn has the atomic mass (mₐ) of 118,71u which means:
356.13/118.71=3 atoms of Sn
The mass number (symbol A) also called atomic mass number or nucleon number is the total number of protons and neutrons in an atomic nucleus. It determines the atomic mass of atoms and it is in the periodic table.