How many Vusers can run on a given computer
There are several factors that can affect the scalability of a script, and the number of Vusers that can be run, it is not always possible to determine the hardware requirements to run a given number of Vusers.

Solution: Determining the number of Vusers that can run on a Load Generator
In order to determine the number of Vusers that can be run on a given amount of hardware a few issues should be analyzed. Because there are several factors that can affect the scalability of a script it is not always possible to determine the hardware requirements to run an exact number of Vusers. The best approach is to run some tests to verify empirically the resource requirements of the specific scripts being used during a test.

The primary factors affecting the script execution are Memory and CPU Utilization. Other factors such as Network Bandwidth and Hard Disk speed can become factors but in virtually all cases the core issue will be Memory and CPU related.

In order to determine the number of Vusers that will run on a given Load Generator machine it is best to run some tests from the LoadRunner Controller. Create a scenario using the scripts in question where they are run on a remote Load Generator using a slow ramp up of Vusers. During this process monitor the Load Generator to see what Memory and CPU resources are used as the load is increased.

Note: In this case, the "load" that is being talked about is the client side processing load on the Load Generator machine itself and not the load on the Server (which is what is normally measured during an actual load test).
Using this method an accurate estimation may be made about how many Vusers can be run on that machine. Also some extrapolation may be done to determine additional hardware requirements for additional Vusers.

It is also wise to monitor the Load Generator machines during the actual test to ensure they are not a bottleneck in the test (as that would invalidate the test results anyway).
Some rules of thumb for monitoring load would be

• CPU usage should stay below ~80-90% continuous usage.
• Memory usage should stay within the physical limits of the machines memory.

This means that the "Commit Charge Total" should be less than the "Physical Memory Total" which will ensures that minimal paging is done to simulate physical memory.

Some additional notes regarding other factors relating to Vuser performance
Run-Time Settings That Affect Number of Vusers

When a test is run, the Controller launches a process called mmdrv.exe to actually run the Vusers. Normally Vusers are run as "threads" inside this process. This option can be changed in the Run-Time Settings under the "General" tab. Running Vusers as threads has the advantage of running multiple Vuser threads per mmdrv.exe driver process, which is more memory efficient than running each Vuser in its own driver process (as a "process"). In some circumstances (usually for stability), it can be desirable to run each Vuser in its own mmdrv.exe process. The key disadvantage to this method is that more Memory and CPU will be used as numerous mmdrv.exe processes are launched.

If you have a version that is prior to LaodRunner7.6, in addition to le to set the Vusers to run as either a process or a thread it is also possible to change the number of threads that will run in each mmdrv.exe process. This option can be changed in Tools -> [Expert Mode] Options under the Agent tab. Here there is a setting called "Max threads for Driver." This setting is by default 50 for most Vuser types. This is an adequate setting for most cases. If desired, the number can be changed, usually setting a lower number would be preferred to have some additional stability of Vusers (as fewer Vusers share memory address space). If one mmdrv.exe process crashes during the test run it will only affect those number of Vusers that it is controlling.

Windows NT and Windows 2000 Application Performance Boost Settings
If a another application like a screensaver, disk defragmenter, or other utility executes on the Load Generator during a scenario execution the Vusers may run slower, time out, etc.
Windows NT and Windows 2000 have settings affecting how programs are assigned time to the CPU which can affect performance. Ideally no other utilities are running during the test run to interfere. If necessary, changes can be made to the System Performance properties that can help the Vusers run at peak performance.

If your Load Generator machine is Windows 2000, configure the following to improve performance and run more Virtual Users per machine:
In the Control Panel Systems dialog box, click the Advanced tab, and make sure that the Background Services option is selected in the Performance Option section.
In Windows NT the same setting can be made in the Application Performance section of the Performance Option in the System Properties. Set Performance boost to "None."