Specifically there are examples for
- Average within centroid distance
- Cluster density
- Sum of squares item distribution
- Gini item distribution
Interpreting the shape of these graphs is complex and the subject for another day. In this case, the "right" answer is 8 and the measures don't contradict this.
As usual, the answer does not appear by magic and clustering requires a human to look at the results but the performance measures give a helping hand to focus attention to important areas.