weka- cluster k means - choose the best number of clusters

#1
thanks for your advance for helping this.

I have changed the various cluster and choose the minimum percentage of incorrectly clustered instances. Report the best number of clusters and explain the reason for getting the best number of clusters

my question: the best number of clusters is the minimum of incorrect cluster?
if so what is the reason chose incorrect cluster why not correct cluster?
 
#3
sorry I am not using sas using kmeans cluster in weka and cluster mode class to cluster evaluation. i got incorrect instances using different classes. my question is why we need to take the best number of cluster