While there are no hard and fast rules for Directory Server capacity planning, actively monitoring the directory is essential to ensure that performance metrics are met. When the system is not meeting these metrics, then it is time to add an additional directory consumer. Typically, you will want to monitor:
Requests per second
Evaluate the above metrics against a target response time of 10 Milliseconds. The IOWAIT should not exceed 10 Milliseconds, and the sum of the CPU utilization in this tier should not exceed 70 percent.