What is the limit to how many services can run on a single instance of ArcGIS Server?

The correct answer highlights that the limit on the number of services that can run on a single instance of ArcGIS Server is primarily determined by the server's capacity and how well it has been tuned for performance.

In practice, while there is no hard-coded maximum number of services that can be hosted, the actual effective number depends greatly on the underlying hardware resources, such as CPU, memory, and disk I/O. If the server is under-provisioned, attempting to run too many services simultaneously can lead to degraded performance or even service outages. Thus, administrators need to monitor the resource usage and optimize performance based on their specific implementations and workloads.

This approach allows for greater scalability. It encourages best practices in managing server resources, ensuring that services are provided efficiently without compromising server performance.