This post shows shows you how to use marker clusters function of the control to display a large number of markers on a map. You can use the MarkerClusterer functionality to combine markers of close proximity into clusters, and simplify the display of markers on the map. The number on a cluster indicates how many markers it contains. Notice that as you zoom into any of the cluster locations, the number on the cluster decreases, and you begin to see the individual markers on the map. Zooming out of the map consolidates the markers into clusters again.
To see how marker clustering works, view the map below.