Hey Amy,
It's difficult to give specific best practices related to performance due to the large differences between data, service usage, and environment.
With that in mind, a single map service should be adequate in most cases to share one feature class for multiple uses. The number of instances will correlate with the number of cores the service runs on. This can be visualized by opening Task Manager on the server and looking at the ArcSOC.exe processes under the Processes tab. Turning on the Command Line column will help associate each process tied to a specific map service. If your map service is set to a max/min of 2 instances, then there should be 2 ArcSOC.exe processes.
The next thing to keep in mind is how many simultaneous users you'll have. I could have a simple web application with one map service and 20 users, but I won't see any performance issues because those users would have a hard time forcing simultaneous requests. From the server's perspective, a request will come in, it will handle that using one of the available instances, and it will return a response. That request/response typically occurs in fractions of a second. As long as the response time is low, your server should not be burdened as the user base grows. The statistics in Server Manager under the Logs tab should give a clearer picture of what is happening in this arena over time.
If any of this information is unclear let me know and we can expand on it.
Thanks,
Scott
