Well, for those who might be interested, I found my answers.
Inertia is a measure of variation, or "spread". When you plot the explained inertia (intragroup sum of squares / total sum of squares) versus the number of clusters you usually notice an "elbow", a big curvature (or a peak in the second derivative) at some point. This is called the elbow criterion and it helps determine when it is no longer necessary to add more clusters. However, there may be many peaks and so this requires critical analysis.