The Administrator app can be used to inspect and manage the data for an Alfresco Process Services Process Engine (or cluster of engines). It also is used for cluster configuration and monitoring. It is distributed as a separate web application (WAR file).
Typically, there is one single Alfresco Process Services Administrator application for multiple environments (for example, development, testing, production, and so on), which is accessed by a handful of users (system administrators). As such, it is generally not necessary to have multiple instances of this application running (although it is certainly technically possible).
The Process Engine is cluster-enabled in the sense that, together with the Alfresco Process Services Administrator, a user can configure and monitor a cluster (or multiple different clusters) through a graphical user interface. The clustered engines will use the same configuration and will report metrics and status back to the Alfresco Process Services Administrator where they are displayed.