When you leave the Number of Clusters parameter blank, the tool will evaluate the optimal number of clusters for you. The tool will solve for 2, 3, 4, or up to 15 clusters. This process can take a while to complete, especially when there are many Analysis Fields or the number of features is large.
Use some other strategy for deciding how many clusters to create. Once you have identified the optimal number of clusters for a fixed set of features, Analysis Fields, and particular Spatial Constraints, you will not need to repeat this evaluation step. If you change the Analysis Fields, Spatial Constraints, or Input Features, however, the optimal number of clusters (as determined by the pseudo F-Statistic) might change.