What is the oxidation number of N in NH4(H2PO4)?

NH₄H₂PO₄ consists of NH₄⁺ and H₂PO₄⁻ ions.

In NH₄⁺:
The oxidation number of H = -1
The total oxidation of NH₄⁺ ion = Charge on the ion = +1
The oxidation number of N = (-1) - 4*(+1) = -3

The oxidation number of N in NH₄H₂PO₄ = The oxidation number of N in NH₄⁺ = -3

The oxidation number of nitrogen in ammonia or ammonium ion is -3 because nitrogen in ammonia is assigned an oxidation state of -3. Its a convention which everyone follows.

You could argue that NH3 is nitrogen(III) hydride and assign hydrogen an oxidation state of -1, making nitrogen have an oxidation state of +3.

You should realize that oxidation states can be arbitrarily assigned numbers. If we arbitrarily assign H an oxidation state of +1, then we can assign oxidation states to all the other elements. The other assumption is that for a neutral compound, the sum of the oxidation states is zero. In addition, the sum of the oxidation states is the charge on an ion.