Clusters are an option that joins nearby trackers together when displayed on the map, this makes it easier to see them. Essentially, nearby objects overlap when you zoom out, so if you have more than two quite close and zoom out your viewport with Clusters enabled, they will join together and show as a number. The more objects you have in proximity, the more likely they are to join together by extending your view, the larger the number becomes. To disable this behaviour, deselect "Clusters" from the map action buttons on the left.