You will need to use your domain knowledge when picking the number of clusters. I often pick colour clusters from the Hotspot menu and in a different platform such as PCA, graph builder or multivariate I look to see how the patterns change when I alter the number of clusters. Two way clustering can also help with this choice.
The 'proper' tool to help assess the cluster number is the distance graph below the dendrogram. You are looking for where this levels off.
