(NH4)2C2O4 is the chemical formula ammonium oxalate.
Di-ammonium oxalate
16
Your compound's chemical formula is (NH4)2Cr2O7 - so you can see that it contains 2 moles of NH4+ per mole of the salt. So in 7 moles you have 14 moles of ammonium, containing 14 moles of N atoms which means that you have (1 mole = 6,022*10^23 atoms) 14 mols * 6,022*10^23 atoms/mole = 8,43*10^23 atoms