how can we find the valency of polyatomic ions like SO4, SO3, CO3,PO4 etc.?

Try to remember these ions in acidic form and count the number of proton it hold, it will give the valency of ion.

For example SO₄ come from H₂SO₄ therefore it has valency of 2, similarly SO₃ is a neutral molecule therefore has zero valency, CO₃ has valency of 2 and PO₄ has the valency of 3