Just use the subscripts
You have 4 hydrogens in one NH4 compound, and you have 3 NH4, so that gives you 12 H and 3 N.
You also have a coefficient of 2 in front of (NH4)3, so that means you would multiply 12 H atoms by 2 and 3 N by 2, giving you 24 hydrogen atoms and 6 nitrogen atoms.
You also have 1 Phosphorus atom, and 4 oxygen atoms, so now add all your individual atoms up:
24 hydrogen + 6 Nitrogen + 1 Phosphorus + 4 Oxygen = 35 atoms in total