An atom of potassium has an atomic mass of 39 amu and an atomic number of 19. it therefore has neutrons in its nucleus.

Atomic Mass =  # of Protons   +   # of Neutrons
                  
   but  # of Protons = atomic number

∴  # of Neutrons = Atomic Mass  -  Atomic Number 
                           =  39 - 19
                           =  20

Thus a potassium atom with of mass 39 amu has 20 neutrons.