Hello !
The number to the right of each element represents the number of the atoms of that element.
PO4 is taken twice so you will multiply 1 and 4 by 2.
Now we have 1*2=2 atoms of P
To the left,there is 2,so you will multiply all the elements by 2.It means that you'll multiply 2 (atoms of P) by 2 :
2*2=4 atoms of phosphorus
ANSWER : 4