In a Pourbaix diagram (E-pH diagram) where the regions are given , but the assignment of these to the elements we have is not done yet , how can the oxidation numbers helps me out assigning each element in its appropriate region?
If you do not have a list of the species that belong in the Pourbaix diagram, then it is not possible to accurately predict what species occur where, unless you have extensive experience with the system in question. If you have a list of species and need to place them in the appropriate regions of a Pourbaix diagram (fill in the blank style), then the higher oxidation state species occur at the top of the diagram and the more basic (e.g. OH containing) species occur towards the right.
If you have Latimer diagrams of all the species in the Pourbaix diagram (this will require the Latimer diagrams for both acidic and basic conditions, typically) then you can more accurately predict the structure of a Pourbaix diagram by (a) writing balanced half reactions and (b) adjusting them for the appropriate pH using the Nernst equation.