Look at your unbalanced equation:

Al^3+ + Na3PO4 → Na^+ + AlPO4

How do I use the ratio of AlPO4 to Al3+ to determine the number of moles of aluminum ions?

1 mole of Al³+

Explanation:

Balanced chemical equation;

Al³⁺ + Na₃PO₄ → 3Na⁺ + AlPO₄

From the above chemical equation, it is evident that this is displacement reaction which is a reaction involving one atom or charge displacing another one in the chemical reaction.

In this reaction, Al³⁺ has a charge of +3 while Na⁺ has a charge of +1. When Al³⁺ displaces Na⁺, it meets an anion of PO₄³⁻ and since both charges are equal, i.e +3 and -3, there's is a mutual exchange of electrons and bond formation occurs.

The ratio of AlPO₄ to Al³⁺ is 1 : 1

I.e 1 mole of Al³⁺ produce 1 mole of AlPO₄

im pretty sure its b