Each server listed on the Overview page includes the number of instances; for example, 42 app instances. The instance count indicates the maximum number of open file descriptors used by the PHP agent in the last reported minute. These file descriptors are open I/O sockets and reflect the agent/daemon communication channels.
The number of instances will increase if command line, background, or external processes are running. The number also may be influenced by heavy throughput. For example, an Apache server may be configured to grow the connection pool to accommodate increase load.
A difference in the expected values between servers may reflect background tasks being run on one server but not the other. An unexpected count also may indicate a misconfigured load balancer.