In the complex $[M(en)_2(C_2O_4)]Cl$:

1. Coordination Number (CN): The ligands are ethylenediamine ($en$), which is a neutral didentate ligand, and oxalate ($C_2O_4^{2-}$), which is an anionic didentate ligand. Total number of donor atoms bound to the metal $M = (2 \times 2) + (1 \times 2) = 6$. Therefore, the coordination number is 6.
2. Oxidation Number (ON): Let the oxidation state of metal M be $x$. The overall charge on the complex cation $[M(en)_2(C_2O_4)]^+$ is $+1$ (to balance the $Cl^-$ counter ion). So, $x + 2(0) + 1(-2) = +1 \Rightarrow x - 2 = +1 \Rightarrow x = +3$. The oxidation number is 3.

The sum of the coordination number and oxidation number is $6 + 3 = 9$.