In ammonium - phosphomolybdate, the oxidation state of Mo is +...

Phosphomolybdate is an anion containing both phosphorus and molybdenum, with the chemical formula ${\left({NH}_{4}\right)}_{3}{PO}_{4} \cdot 12{MoO}_{3}$.

The ammonium ion has a total charge of $+ 3$, and the phosphate ion has a total charge of $- 3$. Therefore, the $12{MoO}_{3}$ units in the compound must have a total charge of $+ 3 - ( - 3 ) = + 6$.

Each ${MoO}_{3}$ unit contains one $Mo$ atom and three $O$ atoms. The total oxidation state of the three $O$ atoms is $- 6$, since each $O$ atom has an oxidation state of $- 2$.

So,

$Mo + ( - 6 ) = 0Mo = + 6$