To run a deployment, Serena Release Automation requires an agent (resource) or proxy agent on the target machine. Typically, an agent is installed in every environment that an application passes through. A typical production pipeline might be, say, SIT, UAT, PROD (the application passes through two testing environments before reaching production). In this scenario, at least three agents need to be installed–one per environment. If different components run on different machines within a given environment, you might want to install multiple agents in that environment.
Whether you need one or multiple resources per environment is determined by your current infrastructure, deployment procedures, and other requirements. Many Serena Release Automation users have significant differences among environments–in SIT you might need to deploy a component to one machine, while in UAT you might need to deploy the component to multiple machines. You could, for example, configure sub-groups for the single agent in the SIT environment and then set up individual resources for each agent in the UAT environment.