How many resonance structures can be drawn for ##"PO"_4^(3-)##?

You can draw five resonance structures for ##"PO"_4^(3-)##, but one of them is a minor contributor to the resonance hybrid.

If you start by drawing four oxygen atoms single bonded to a phosphorus atom and give every atom an octet, you get Structure R in the diagram below.

This is not a "good" structure , because it has five atoms with formal charges.

We can reduce the number of charges by moving electrons to convert a ##"P-O"## single bond to a ##"P=O"## double bond.

There are four ##"P-O"## bonds, so we can draw four structures: S, T, U, and V.

These are all equivalent and are the major contributors to the resonance hybrid.

Hence, the actual structure is a resonance hybrid of all five structures, with : S, T, U, and V being the major contributor and R a minor contributor.