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The amount of concurrent users on a session host depends on the size of the VM and the applications that are used.
To test if the sizing is appropriate, we recommend to use at least one VM and let so many users work on that VM as it is intended. This will give you most realistic data.
You can also look at artificial load simulators like LoginVSI. However, those might need to be customized to use the apps that you are going to use.
Hope that helps.
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