Zinc is a Transition Metal. You can only know how many valence electrons Zinc loses by the valence electrons of Phosphorous. Because each Phosphorus atom needs 3 more electrons to become stable like Noble Gas. Meaning P wants a total of 8 valence electrons. So if it has 5, it needs 3 more to have a total of 8.
Zn3P2 is an Ionic compound. In the compound, there are 3 atoms of Zn, and 2 of P. If 1 atom of P needs 3 valence electrons, then 2 atoms of P will need 6 valence electrons. There are 3 atoms of Zn, that means each Zinc atom has 2 valence electrons. So, the number of atoms times the number of valence electrons is 6 valence electrons. 3(2) = 6. This 6 valence electron goes to P2 because P2 needs 6 valence electrons. So the net charge is 0.
So, based on all of those each Zinc atom loses 2 valence electrons.