# How many oxygens are there in Mg_3(PO_4)_2?

In the one formula unit of $M {g}_{3} {\left(P {O}_{4}\right)}_{2}$ there are $8$ individual oxygen atoms.
All I have done is simply expand the formula using the multiplier on the brackets. One formula unit of $M {g}_{3} {\left(P {O}_{4}\right)}_{2}$ contains $3$ $\text{magnesium atoms}$, $2$ $\text{phosphorus atoms}$, and $3$ $\text{magnesium atoms}$.
Can you give the atom number breakdown of the salt magnesium biphosphate, $M g \left(H P {O}_{4}\right)$? Why is this a 1:1 salt and not 3:2?