Performance Monitor can reveal trends when you watch and record data. By using performance counters to establish a baseline to measure against, the requirements for sizing to meet demand can be better predicted.
Although actual performance numbers will be determined by operation experience, as a general rule, a single ILS server is designed to scale to tens of thousands of users and handle millions of queries per day. For more information about scaling ILS, see Chapter 3 in the Internet Locator Server Operations Guide.