Start with the preset argument number value, if you notice in the results that two or more clusters have the same name (cluster name) then you can repeat the processing with a smaller number of clusters in order to merge those with a common name (argument). On the contrary, if you notice elements (phrases) within a cluster that you think might constitute a separate cluster, repeat the processing by increasing the number of clusters. Finelly you will get a set of clusters each with a different name (argument) ;)