atomicity is the total number of atoms present in one molecule of an element, compound or a substance. For example in O2, there are 2 atoms of oxygen.
similarly ozone has 3 atoms in each molecule so its atomicity is 3.
A sample of ammonium phosphate, (NH4)3PO4 , contains 6 moles of hydrogen atoms. The number of moles of oxygen atoms in the sample is :