So, the output of hierarchichal clustering results can be determined either by number of clusters, or by the a distance thereshold to cut the tree at that threshold. However, scikit learn only supports one way!

class sklearn.cluster.AgglomerativeClustering(n_clusters=2, ...

As a suggestion, is it possible to add the other option, to give distance_threshold as input argument and get as many clusters are created in the output! So, one of this argument should be given, either n_clusters or the threshold, not both!