how many electrons does P and O have in PO4^3-
Draw the Kekule/Lewis/Dot-&-Cross structural formula for the
phosphate(V) ion.
Counting only the valence electrons (you go ahead and add inner
shell electrons yourself), notice that phosphorus has a double bond
with one oxygen atom, and single bonds with three other
oxygen atoms. Notice that phosphorus has expanded its octet, and
has no formal charge on it. The double bonded oxygen (with 2 bond
pairs, 2 lone pairs) has a stable octet and no formal charge, while
the single bonded oxygens (each with 1 bond pair, 3 lone
pairs) have stable octets and a -ve formal charge (on each
oxygen). Hence phosphate(V) ion is tri-negative (recall that ionic
charge = sum of all formal charges).
See its Kekule structure in Wikipedia :
http://en.wikipedia.org/wiki/Phosphate