what is meant by coordination number ? With appropriate examples.

For ionic compounds the bonding forces are electrostatic and therefore omni-directional.  The bonding forces should be maximized by packing as many cations around each anion, and as many cations around each anion as is possible.  The number of nearest neighbor ions of opposite charge is called the coordination number.  We must realize however that the coordination numbers are constrained by the stoichiometry of the compound and by the sizes of the atoms.

e.g. For sodium chloride, Na⁺Cl^-, there are 6 anions around each cation (coordination number Na = 6); because of the 1:1 stoichiometry there must also be 6 Na cations around each Cl anion.