The number of virtual users that can be run from a single system is dependent on several variables like Agent system characteristics (CPU, memory, network), workload being run (OLTP, DSS, Capture/Replay, etc.), and the server being tested against. For most typical tests and Agent systems you can get around 500 virtual users per machine. Due to the nature of the capture/replay test you may only get between 200 - 300 virtual users on a Agent system. A typical Agent system is a dual or quad core with at least 1G of memory.
So let me explain. If the workload you are running, and the server performance, causes the transactions you are executing to respond quickly than the number of Agent system required increases since as the response time is faster, the Agent CPU power needs to be equal to that of the server machine. As the response time slows, more virtual users can be run on the Agent machines since less CPU power is required. For capture/replay tests since most of the time these tests are reading in the SQL to execute from XML metadata files, the Agent machine needs more CPU power in order to keep up with load and therefore you will be able to have less virtual users on the Agent systems.
I hope this helps as a guideline.