Here's how to determine the subscript of ammonium in the compound formed with phosphate:

1. Understand the Ions:

* Ammonium Ion (NH₄⁺): This is a polyatomic cation with a +1 charge.

* Phosphate Ion (PO₄³⁻): This is a polyatomic anion with a -3 charge.

2. Balancing Charges:

To form a neutral compound, the charges of the ions must balance. Since phosphate has a -3 charge, we need three ammonium ions (each with a +1 charge) to neutralize it.

3. Subscript:

Since we need three ammonium ions, the subscript for ammonium in the compound will be 3.

4. Formula:

The final formula for the compound is (NH₄)₃PO₄.