I want to group my dataset using clustering technique. I apply k-means and used Dunn index and  Silhouette Coefficient  for the validation. now I want to know what should be the optimal cluster size based on the Dunn index. For your reference i am uploading the DI and SC plot. 

The point is that if I keep on increasing the cluster no's the DI value is getting higher after 7. The minimum value in figure is 5. So can't we take 5 as the min no of cluster possible.  

Please suggest what should be the cluster size i need to consider for this plot.

thanks 
 
[![enter image description here][1]][1]


[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/ngv4S.jpg
  [2]: https://i.sstatic.net/v6CJq.jpg