Having these different approaches to determine a convenient value of k (number of clusters) should help to narrow down the options for k, however, we cannot make sure the value we select finally is the best or not. For example, if silhouette measure is closer to 1 when k=5 and for Inertia we have the "elbow" or "knee" around k=4 to k=6, probably one of the values of 4, 5, or 6 might be a good value for the number of clusters. These two methods together help us have some estimations for the number of clusters but they do not guarantee it.