What is the coordination number of the central metal ion in [co(c2o4)2(oh)2] 3- ?

             The coordination number of the central metal ion in [Co(C₂O₄)2(OH)₂]⁻³  is 6.

Explanation:
                   Coordination number is defined as the number of neutral atoms or ions surrounding the central metal atom.
                   In given case there are four ligands surrounding the Co central atom. 
                   1) Oxalate Ion:
                                          Oxalate ion has two negative charges (shown below). So each oxalate makes two coordinate covalent bonds with central metal atom and four coordinate covalent bond by two Oxalate Ions.

                   2) Hydroxyl Ion:
                                            Hydroxyl ion has one negative charge. Hence it makes one coordinate covalent bond with Co atom.

It is called a neutralization reaction

Cr(oh)3 -> cr(3+) + 3oh-