Within an example vSphere environment, there are 7 clusters, however, when navigating to the Capacity Planning dashboard (VMware Environment | Capacity Planning) there are only 4 clusters listed.
This is by design. Only clusters with multiple shared resources are listed in the "Select Cluster" list. If a particular cluster only contains a single host, this will not be listed within the "Select Cluster" list. Using the Capacity Planning "Scenario Modeller" should help provide a solution to this.