You're right that agglomerative clustering (unlike K-means) does not inherently need to know the # of clusters in advance. However, it still needs some sort of

You're right that agglomerative clustering (unlike K-means) does not inherently need to know the # of clusters in advance. However, it still needs some sort of termination criterium, and # of clusters is one possible criterium. Since lsm operates in a transformed space, other commonly used criteria like cluster distance may not be as convenient for the user to express.