To determine the total number of atoms in one molecule of (NH4)3PO4, we need to multiply the number of atoms of each element by the number of atoms of that element present in the compound, and then add the results together.

In one molecule of (NH4)3PO4, we have:

- 3 nitrogen (N) atoms from the 3 NH4+ ions

- 12 hydrogen (H) atoms from the 3 NH4+ ions

- 1 phosphorus (P) atom from the PO43- ion

- 4 oxygen (O) atoms from the PO43- ion

So, the total number of atoms in one molecule of (NH4)3PO4 is:

3 N atoms + 12 H atoms + 1 P atom + 4 O atoms = 20 atoms

Therefore, there are 20 atoms in one molecule of (NH4)3PO4.