N:  is  number of things, i.e. atoms, molecules or particles

N_A = 6.022 x 10²³ mol^-1

n is number of moles. Unit:  mol

m is mass. Unit: g

M is the molar mass (molecular mass, atomic mass or formula mass - older designations: molecular weight, atomic weight.)  Units g mol^-1

r is the density usually with units of g mL^-1 or equilantly kg L^-1

n is the reaction stoichiometric number (coefficients used in the balanced reaction) and is an exact number.