There is one phosphorus atom in the chemical formula H3PO4.

Here's why:

* H3PO4 represents the molecule of phosphoric acid.

* The subscript "4" after the "P" indicates that there are four oxygen atoms.

* The subscript "3" after the "H" indicates that there are three hydrogen atoms.

* There is no subscript after the "P", meaning there is only one phosphorus atom.