Could you please clarify your question and perhaps provide some details on your use case?
The OneAgent for AppMon is currently GA for some technology types and is in BETA for others. Due to the BETA status of some tech types, it is recommended that you do not yet utilize the OneAgent in production and that you utilize it in non-production to test if it works with your custom dashboards, configurations, etc.
Here are more details about the OneAgent for AppMon.
That link also shows the differences between the AppMon and Dynatrace OneAgents. Please note that the Dynatrace OneAgent should be configured to Dynatrace SaaS/Managed systems and not AppMon.
Hope this helps!
Did you mean to say you wish to instrument only a single server/instance for Appmon?
If yes, then only single instance will not give you 100% visibility into your application because the purepath data will not be captured for the hits going on other servers. So it is strongly recommended you instrument every instance to get clear picture of application health esp. the Transaction Flow and bottlenecks.