There is no specific cap to the number of virtual users that ZebraTester can support. The limit is defined, rather, by limits on the hardware being used. All product components are implemented in a fully multithreaded manner. However, the underlying OS of a single executing agent can still be overloaded if too many concurrent virtual users are executed at once. In most common cases of agent overload, the CPU utilization of the agent is nearly 100%. In this case, the measured response times are not valid because the system itself is overloaded.

To accommodate for these limitations, ZebraTester supports the combination of an unlimited number of Exec Agents in a single cluster so that an unlimited number of virtual users can be reached. In other words, the maximum number of virtual users is only limited by the strength and number of Exec Agents. ZebraTester supports execution of cluster jobs transparently and automatically merges the measured results of the corresponding Exec Agents into a united load test result. Several cluster jobs can run in parallel by using the same or a different collection of Exec Agents. Exec Agent clusters can be created by using a combination of both local and remote Windows and Unix systems.